Good thread topic. I just discovered this, and I'm almost in the same situation as you.
What I did was to buy 2 1084 Amiga monitors from QXL, get a small TV, a switcher for the tv-cable, so now I can run 4 computers on 3 screens. I have a Vic-20, two C64s, a C128 and an Amiga which I want up and running at any time.
This really didn't cost me much, and I'm gonna build a setup where the computers can sort of slide under the monitors, using planks and stuff.
"On the Edge - The History of Commodore" and "Digital Retro" are also part of the collection, as I see these books as essentials.
I also like to have a few CDs with nostalgic music at the side. Now I need a system with some speakers and a switch so I can play MODs and SIDS.
Some old Nintendo Game & Watch handhelds are also nice to have lying around.
Finally, my plan is to have an old PC laptop to use as a server for all the computers using serial and XE-1541 cable for transfering stuff.