Zeeflyboy is right, you should be getting a better spindle than the Dremel. I have a Dremel tool also, but I think the air flow is not the worst with that, but the extreme noise and the crappy bearings. They are also pretty weak. So, my opinion is that you should add a real spindle and a VFD to your budget. Of course, even if you give up on carbon fibre, adding a dust shoe is a very good idea, regardless which sort of spindle you use or which material you mill. Regarding dust and carbon fibre milling, one way of avoiding the dust is milling in a water bed. Have seen some people doing that, but you need to build the rig, so it may not be that simple to do.

Personally I gave up on Mach3 and using UCCNC from CNC Drive. I am also using their UC300ETH, which works also with Mach3. My drivers are DQ542MA and they work very well.