Neil, your question is pretty general, and it all comes down to the components installed, and these are driven by $$.
Any set of ground ballscrews will outperform rolled screws for accuracy and torque requirements, and these will usually outperform typical ACME screws.
Servos are generally better than steppers. Ways can be cylindrical bushing, THK-style rails/trucks, or iron dovetail. Each of these has advantages and disadvantages relative to speed and accuracy.
The spindle is really the heart of the machine and I don't think we "talk spindles" as much as we should on this site, maybe because there really aren't that many smaller spindles available. You go from modest Sherline setups, and mini-mill R8 heads, and jump right to big $$ HF spindles, and big iron Setco and Gilman ISO30 and up taper spindles. I really wish there was more in that big gap.
I recommend you analyze it backwards. Envision the product you want to make. Route wood? Make steam engines, or IC engines? Jewelry, molds? Or aerospace stuff which needs to be +/-0.0001"? Next, you take the $$ available. $500 is not going to produce a machine which will make the aerospace stuff at 200 ipm, so without the $$ available, you may have to reject certain machines and components.
So given the $$ available, and the product you want to make, I'd start at the spindle and build the machine around that, considering the performance of the installed components. I'd not bother installing ground ballscrews if your spindle has evil runout, or the ways have slop of .005". Match the precision of the components with other, mating subsystems.