A few years ago (1999) I has something similar made - but 8 x 6 feet
only FYI. Wooden shed, extra wide door (for tables to get in and
out), insulated. Had a frost free base laid (bricks and concreete
AIUI!), shed went on that. Paid a sparky to puit in the leccy, then
BT put in a phone line. It can be chilly in the winter - but I first
had a electric radiator on a timer in there to keep the damp/cold at
bay (and believe me in 8 x 6 feet it didn't take long for it to be
swealtering! Now I have several systems running 24 x 7 I don;t bother
with the heater as the systems create enough heat top prevent damp in
the paperwork. A small fan heater does for if I get chilly.

Total cost about a grand - but its really a case of how long is a
piece of string. The insulation is (for me) a must, otherwose all you
have is a woodern shell against the elements. Security isn;t too big
a deal as the only system that is "KEY" and worth £££ is my laptop
which lives in the house with me. You could make of security what you
will of course - door bolt, internal hinge bolts - no windows would be
best of course ! Then you could use wall brackets etc to tether
equipment to - a client of mine has steel cages that are fixed to the
floor within which systems are locked and you could probably finf
something similar if you had a concrret base to afix them to.

As for size you might get away with smaller than you think - don;t
forget you have 5-6 feet of height to play with also. A smaller room
will heat easier (maybe the systems will be enough on their own) and
of course be cheaper to buy. you'd be surprised for instance how much
you can fit in 8x6 feet - I've a small network running with a desk and
a bookshelf/filing shelf easily, with a small window.

ian