Then that is what you should use. "Period correctness" is meaningless if it's a machine that's too cumbersome to bother with. Adding a PS/2 mouse port isn't nearly as easy as it sounds.
I likewise disagree that a 486 is absolutely the way to go. The ideal would actually be a fast 486 or Pentium with a turbo button, but you should easily be able to use a slowdown program (or BIOS trickery) to run programs for which a 200 MHz Pentium is too fast. It is good to have that speed available to you when you need it.
And yes, a PCI video card will work just fine from DOS, and so will an AGP card! The only thing you really have to worry about as far as DOS programs are concerned is VESA compatibility, which isn't much of an issue.