I think that rigidity and accuracy is CONSIDERABLY more important than speed, especially for aircraft. In your case, balsa is not an issue, but carbon fibre definitely is tough and demands a really rigid machine. I would not build a CNC from anything less than aluminium. Belt drive sound good in theory, but I think that in reality, if you want quality and accuracy and no backlash then you can't just take any belt... If I'd plan to build this machine I'd still use ball screws and ball nuts. I don't think you will have issues with speed, unless you have unrealistic speed dreams. With steppers you should be able to get about 7000 to 9000 mm/min maximum speed, at least if you are using fixed gantry moving table design (which is better regarding rigidity), and probably something between 4000 to 6000 mm/min if you go for moving gantry design. Remember that maximum speed is normally not the milling speed, that depends on the material and many other things, so in all honesty, I don't think there is any advantage in belt drive in your case, even if you are limited in top speed.

...but I think the biggest issue is your budget limits. I really think that you have to double that (or maybe even more) to get a good enough machine to mill carbon fibre.