The size works to your advantage. The only thing I'd worry about in this case is Z axis hight, you want to be absolutely certain that 3" is enough. If you add a spoil board and fixturing you can quickly have a very tight fit between the work piece and gantry. It might be good enough for what you want to do but you need to verify that in your mind.
On a small machine, especially if the gantry span is narrow, you can get away with small tubing cross section wise. Even so your gantry beam will likely need to be of a larger cross section than other frame members to separate the Y axis linear bearings enough to provide stiff support for the X axis.
I would think that C would be good enough for what you have described so far. Your budget is a factor here, if running open loop higher precision lead screws do help for precision over a given distance. This is important to understand but you need to have an idea of what your expectations are and what your budget will allow. Frankly if servos are too rich for you right now Im not sure pursuing high precision components is an ideal approach.With this in mind, should I even consider the higher precision grades for the rails/ballscrews, or is "C" more than enough? What size rails does the reader recommend?
Do realize that ball screws are subject to wear and eventual replacement. They can also be replaced at anytime if you decide you would benefit for higher precision.
Nail down your machine design, size and construction material first, then work on the controls issues.
I am thinking for such a small machine, 15mm is fine but I am all ears. As far as controllers and steppers go, I have no clue how much I need,other than the fact that I cannot afford servos or linear motors. As far as spindle goes, I know I want better precision than can be afforded by routers, but that's about it.
Thanks for reading!
Highlight to us what you have access to machine shop wise if anything. While epoxy leveling works there are still many things that need to be done on such a machine that would benefit from being able to machine parts. The nature of the machines design impacts the need for machine work.