You have conflicting goals for your limited budget.?

Woods plastics etc are no problem as accurecy isn't critical but PCB works requires accuracy and this means building to higher standard which means higher quality components and these cost money.

Also like John points out the motors you have probably won't help you anyway if your going the threaded rod, skate board bearing and MDF route as efficiency is very very low so friction high.

Cutting aluminium on limited budget is doomed from the start so I'd forget that instantly.!

In a nut shell what you get from CNC is directly proportional to what goes into it and if your limited on budget with limited tools then expect limited results and accurecy.

Whats your budget.?