So, if you had roughly $1500 - $2000 to spend on a setup, which direction would you go?

It depends on what your goals are:

• If the initial $1500 was just a "I want to get my feet wet" (i.e. learn/play with out the rudimentary aspects of controlling a 3 axis machine via a computer, but I was also totally NOT concerned if I was actually creating something "useful" as an output {if it happens, fine... if not and all I created was sawdust... well, who cares, the "learning-journey is the reward"}) ... then I would go with some dirt-cheap set of plans and build my own from scratch (and/or if I had NO tools, then I'd buy the cheapest "kit" I could find).

• On the other hand, if you want to learn but ALSO produce at least some specific or particular useful "thing" (i.e. you need/want something to show as a "product" of your experimentation, and you or your S.O. will be ticked off if you can't) -- then I'd probably by the smallest finished mechanism that would create the final output "thing" I wanted. (And depending on what the size/precision of that "thing" is, I would note this may NOT really be possible for $1500).

• If I was really interested in producing MANY copies/variations of some specific thing (i.e. lots of signs, lots of speaker cases, lots of ___________) then I'd take a more planned approach.

1) First, I would identify what (as specifically as possible) that I was wanting to create, so that I could identify what I was going to need in terms of software and tooling (i.e. router bits, milling bits, etc.) -- and that I could buy the GOOD stuff for that.

2) THEN, I would figure out just how much of a "machine" I could get with the remainder of my current budget.

1. The reason being that the software/tooling can easily transfer from a "small/beginners machine" and still be useful on some second "medium/intermediate" machine or even an expensive "large/professional" grade machine. But a "better" grade machine with inadequate software & crap tooling won't be worth much of anything.

For example if I wanted to produce commercial quality wood/plastic signs, I'd figure that I was at least going to need the "V-Carve Pro" software, a solid router (probably 1/2 shank & name brand like Makita), and a series of appropriate "V" bits, ball bits, spiral & compression bits, etc. -- total for those items is probably well over $1,000, possibly $1,500 (V-Carve Pro alone is $600, even basic spiral/compression bits are easily going to be in the $50+ even $100+ range (each) and to do serious work means buying several).

Or, if I was wanting to cut/carve some type of 3D objects, then I would need to be getting some 3D CAD software, possibly a 3D scanner, etc. (as well as appropriate tooling for the materials -- wood, plastic, aluminum all have their own "bit" needs that are significantly different). This could easily total well over $2,000 (and so which machine would equal "none" because it really isn't doable for a total of $2k or less).

If I was going to be cutting circuit boards... then (derp) I'd need software specific to creating circuit board G-Code, and the spindle & tooling for that (which is probably on the cheap end -- like a "Dremel" and small/cheap bits) -- so I might only have to toss $100..$200 at them.

Conversely, if I was wanting to cut 2D shapes out of wood/plastic, then a lot cheaper less sophisticated software would do (and there are even free alternatives), plus depending on the size of what I was cutting I might need a more powerful router (or could possibly get away with a "zip-saw"), and/or different array of drilling, spiral and/or compression bits. Likewise, I could possibly do this for $100 or less... leaving all the rest for the "machine".

What are the inherent dis/advantages of having a moving gantry versus fixed?

Plenty of other sites discuss this. The main things are total footprint and how much mass you are shifting around -- obviously you want to move the LEAST mass (giving you both better precision AND better speed from the same motive power): so if you are dealing with large/heavy raw material, moving the gantry on the Y axis is simpler; conversely if you are working on small/lightweight materials, then moving the work back and forth on the Y axis is probably better. That said, a moving gantry machine typically has a "locked in" material size, whereas some fixed-gantry designs have "expansion" options that can allow larger Y-axis materials.

Which components govern the final accuracy and tolerance?

Ummm... (not trying to be sarcastic, but)... ALL of them.

Everything is a series of tradeoffs.

Accuracy, tolerance (and precision) are typically -- especially in low-end, hobbyist-grade machines (but to some extent in higher quality & commercial machines as well) inversely related to things like size and speed (and obviously cost); the larger the machine, the less precision one is likely to get, as to have anything like a useful cutting speed (you really don't want to wait a hour for the head to move from one end of the work to the other, do you?), you end up having to "gear up" the mechanism from the drive motor to the motion... and lose precision as a result.

As far as "accuracy" (chiefly meaning "repeatability" in terms of being able to return to the EXACT same spot after mucking about in other locations, the generic "accuracy" in terms of the machine versus things like inches/millimeters is typically adjustable in the program to the motors used) -- but said repeatability is a factor of the drive mechanism as well as whether there is a verification system in place (i.e. does the computer simply assume that the cutting head actually moved 3.65" on the X and 14.73" on the Y when instructed to do so, or is there some feedback/validation going on via an independent sensors... so if there was some binding/slipping the computer is aware and able to compensate).

And then -- if it even HAS a feedback system -- what KIND of system is in place? Is it just going back to zero-zero every other minute? Is it using some encoder to count rotations of the motor and assuming rotation=motion, or is it using some other linear scale encoding & actual "validation" of location.

Feedback & accuracy/precision add significantly to the cost... so how important and just HOW accurate/precise does it really need to be for YOUR application(s)???

Keep in mind that it is one thing for a machine to move back to an exact location 5 times on a single piece, and something entirely different for it to have to go back to it 50x or 500x with different bits (in short, tolerance errors can be/often are cumulative).

IOW, you can build a CNC on the "cheap" that will be REASONABLY accurate and precise (in hobbyist terms, and for certain types of work) even without a feedback system... provided you are willing to risk that some occasional "bump" may entirely ruin a piece every now and then. But the same machine may be entirely unsuited for other types of work.

A couple projects I want to have a go at making are, a custom wooden computer case, and speaker boxes.

Yeah, but -- pardon me for saying so, but even that is not really all that specific.

What kind of work do you want the CNC to do to make those "custom" cases/boxes?

Are you just looking to have it precisely route the outer dimensions and punch a couple of holes/slots, or are you looking to have it do some detailed/fanciful "relief" carving? (i.e. you want a 3D fire-breathing dragon on the side of the case).

Because if you just want to route the outer shape & punch a few holes/slots, then you can probably get away with a relatively cheap router or "zip-saw", a couple spiral bits, cheap/free software, and a generic kit machine that just "goes home" from time to time, and really ISN'T very accurate (or is a bit "sloppy") as far as tolerance/repeatability.

Conversely, if you want it to carve that "beautiful" 3D-ish Chinese Fireball dragon on the side in full-relief and fine detail -- and on some very expensive wood that you will be all "pissed off" about wasting if something goes wrong -- then you're going to need a LOT more sophisticated (and expensive) software, several different router bits (roughing, finish, drilling, perimeter cutting, etc), and a pretty accurate/precise machine.