The dremel might work, if you have one try it. If not don’t buy one for this purpose. Not that it won’t work but it is very far from ideal and actually kind of expensive.
You will never mill a 5’x5′ pcb so if that is the main purpose of your machine you should really do yourself a favor and make it much smaller. I think most PCB mills are about 5″x5″…
If you have not used a cnc mill before you are going to have a very hard time machining aluminum with a 5′ machine. Pcb’s and aluminum parts are at exact opposite ends of the cnc spectrum. Have a look at commercial machines made for these purposes and you will see they are built very different.
If you want an all purpose machine, make it as absolutely small as you can.