<any that you or I can reasonably afford of any speed.  They'll probably
<run until the next ice age, but they cost a fortune to operate (just my
<VS3100 eats a huge amount of power, at least 1.5A@120V), and the

The power requirments of vs2ks and 3100s are about that of the average
PC or lower. Most PCs have power supplies in the 170 to 250W range.
The VS2k power supply is 160w.

<availability of spare parts can be sparse and may take days/weeks to
<find.  At best you could have a couple backups on hand, but geez, how
<much space do you have?  

FREE parts and spares yes.  New stuff from DEC 24-48hours for a price$.
As to space, two xerox paper boxes(~2cuft ea) hold enough boards, drives
and power supplies to keep several Q-bus PDP-11s and vaxen going should 
they ever fail.

Allison