How many machines in the warehouse areactually functional?
Nobody knows. Jeff B. and I have tested many of the common micros and labeled them as working, partially working, or broken. But as for the larger and rarer stuff, it's a mystery!
I really would be interested in a workshop to work on some of the machines
We already do this in a limited sense. Before every workshop, we consider what needs repair among the museum systems and what we're in the mood to repair from the warehouse collection. We also consider who's attending the workshop (because people have unique skills), etc. .... for example we pulled a PDP-8/E out of storage when David Gesswein attended a workshop, and more recently a few of us led by Ian made a working FrankenLisa from four dead ones. We'll definitely need to do more of this with the upcoming museum expansion.
I don't know if I'd want to start with anything super rare
No problem. We wouldn't let you. :) (Certainly nothing personal! Just makes sense that we limit access for rare/exotic stuff to the people who have recent proven experience.)