You do need a Sun keyboard (I use a type 5). They are slightly different than what most people are use to, takes a while to fit in. I haven't seen any PS/2 adapters, but I'm sure there are some out there. But there should be a USB port on the back as well. I've never tried that (and don't think it would work, but worth a try).

You shouldn't need a 13W3 adapter, mine came with a standart ATI Rage 128 PCI VGA card. I haven't tried using any other PCI video cards yet, but just about anything PCI should work.

Solaris 10 does work, but 400MHz with >256MB RAM is not going to make it fun. I decked my Ultra5 out with a Debian SPARC install. BSD is also a decent choice. But it depends on what you want to do with it. I'd say find what makes you happy. Solaris 7 to 9 would be fine.

As for those of you with Classic 32-bit SPARC machines, I would suggest either going with OpenBSD, or Solaris 2.4 - 2.6

Anyhow. I hope that helped.