I can sort of see what you're aimming for. I have one old K6-2 system I keep around because I can use it to run an emulator I have the requires Himem support. And, if I wanted to, I could run older software on it.
But considering the price you're likely to pay, a seperate PC would be so much cheaper. Actually, you said you had one, didn't you?
I think I just threw my last vesa video board away. The oldest thing I have is a Voddo3 2000 and that will do VESA modes so why put up with ISA or Vesa?