Hello and need advice on my first cnc

Hi everyone, I'm completely new to the world of cnc machining and am thinking about giving it a try. I've been an rc helicopter hobbyist for about 5 years now. I like doing my own custom modifications and so far have been limited to doing things by hand with routers, dremels, drills, etc. A cnc would really open up the possibilities for me. I've started learning to use cad and have a pretty good handle on that, but no cnc experience at all. The stepcraft seems like a good machine to start with. Basically all the work I would be doing is with carbon fiber plate, mostly 2mm, and hopefully some aluminum parts. I have a few questions about the capability of the stepcraft and choosing the right one and accessories. After watching some videos, it seems the carbon fiber plate is no issue at all, but aluminum is a bit more challenging and a more rigid mill would be better. The parts I would need are relatively small, and not done as often as carbon fiber, so super slow feed rates and shallow cuts is not really an issue for me. An example of a part would be a bearing block that is maybe 6 or 8mm thick, 40 mm X 40mm square, and a pocket maybe 20mm diameter and 5mm deep milled for a bearing to fit. Later on I would possibly add the 4th axis and try to tackle aluminum blade grips. Basically a 20mm or so cylinder with a fork milled into it.
Would it be realistic to expect the stepcraft to make decent aluminum parts like this? Like I said, speed is not a problem. I'm fine with letting it mill on a blade grip for hours. I realize that there are machines better optimized for milling aluminum parts, but if I can't mill aluminum parts with a machine under 2500 bucks, then I'll just accept that I'm limited to cutting carbon fiber.
Here's my next question, is a smaller size machine stiffer and better for milling aluninum? The 600 would be large enough to cut any carbon fiber plate I need, but the 420 would be big enough for 90% of parts and for bigger ones I'm assuming I can cut most of the part with the sheet hanging over the edge and reposition the part for the rest. Would the 420 be significantly better rigidity for milling aluminum, or would the 600 be nearly as rigid?
I would greatly appreciate any input. Thanks!