Question 5: Finally, given this setup, what cards would you choose,
  and what topology or wiring?  BNC seems the most easily extensible;
  with TP I'd have to get a concentrator (or so I believe) when I go to
  3 peer units.  Any other options?  Do they have fiber network cards in
  the low price range (yet)?

TP is the best investment in in-the-wall wiring terms. Nobody is putting
much anything else into the plates&ducts. If you don't have that as an
issue you can afford to use BNC. However well done TP will run 100mbs
fast ethernet or emerging FDDI-on-copper or ATM or any other sick disease.

If its *YOUR* dollars, its a tough one. If its somebody else's then spend
on UTP and hubbing. Its sensible for >1 years investment.
  
  Question 6: Is getting a VLB or EISA network card on the NetBSD/i486
  machine worth it, if I still have an ISA hd controller? (I assume not)

If the box is working memory-cpu-ether more than to disk, it might be.
if you have VLB, and have 3 slots, then you can expect to upgrade disk
to VLB scsi and get a really nice throughput.
  
  Question 7: I'm looking at getting a new hd controller soon; I have
  an AHA-1542B now.  Should I go with the bustek EISA controller, or
  some VLB controller?  Real experience valued here.  I hear that some
  vlb disk controllers can significantly slow down the CPU.  rumor?

I have a bustek vlb and its fine for me. If somebody explains what
set of benchmarks to run I'll run 'em, its NetBSD current, 16Mb, vlb
ether/video/disk and essentialy quiescent for a few weeks more.

VLB ethernet cards: I got a compex NE2100 compatible, getting it to
boot from a generic kernel is hard, I had to write a special onto
the kc-bt floppy after advice from Charles Hannum (many thanks) but
after a bootstrap glitch its nice.

I think I'd VLB it, and jump to PCI when its mature. I suppose again
if you're spending your own money and a good EISA crate comes up the
decision is harder.

-George