Home CNC Computer of Choice Using Older Components

I have access to a number of older computers, laptops, CRTs, LCDs and
various keyboards, mice, trackballs , etc.

What are the desirable features for a computer that will be used in
building a CNC system?

What speeds, form factor, ISA/PCI/? bus, I/O - serial/parallel/USB,
monitor choice/resolution, etc. should a person use to build a good
computer targeted for the shop environment and usage?

I also have a number of rack mounted computers...is there any advantage
to using them over a desktop or a laptop? They are older and slower but
extremely well built.

Finally how much speed, memory, hard drive does a homemade CNC home
computer need for the popular softwares in use? This will likely decide
which computers are used and which are salavaged.

Thanks

TMT