Don't know if any of this is of help. Location is East Anglia, UK, so temperate and low rainfall.
A friend from Norfolk built the shop, to my design. Fourteen years ago he charged £1600, which included two 100 mile return trips from his home to mine.
My mistake was siting it on slabs on sand/cement. Should have been a reinforced concrete base, TOO LATE NOW!
The aim was to use 8 x 4 sheets wherever possible, (In practice, the centre panels have had to be smaller, to keep within the 10'9" x 6'9" max external dimensions. (Max SWMBO would allow between her beloved bay tree one way, and the fence and patio wall the other.
50mm framing, for walls, with 19mm T & G cladding and 12mm Ply inner, over glass fibre. Having asked for 5 lever lock, he supplied a secondhand Fire Door with a 6 lever lock. This needed a 100mm x 50mm framing to support it, and insulation to match.The roof has 12mm cladding on each side of 50mm framing with glass fibre insulation.
Roof is flat, with 6 inch slope from front to rear, 8' high at front, 7'6" at rear, (door is offset in end wall), to run rain water into a butt for SWMBO garden.
Original Felt roof cover replaced after eleven years with one piece EPDM rubber (From Rubber4Roofs. They do a neat calculator for the quantity required, and supply the edge trims that you specify, with plenty of ring shank nails. Get those in the right place first time; not easy to remove! Order by midday and everything arrives first thing next morning! Just a satisfied customer)
The coldest part is the 18mm Ply floor, (diagonals are within 1mm!) carrying 1" deep plastic mats, redundant from work. Floor is carried on 8" x 2" bearers, to allow legs of a folding crane to pass beneath it.
No windows, for security, and anyway I would only have put shelves across them!
Ventilation is by two fixed vents near floor, and intake by rarely used ex equipment 6" fan near ceiling, all on the back wall. (Terry said, quite correctly,"You'll spend most of your time with the door open"
Front wall carries shelves on 12" centres, (Late bargeboarding from a neighbour's house, plus anywhere else I can squeeze one in, even over the door! Shelf behind the lathe carries Drill Chucks, Faceplate, Ccollet chucks, Steadies, etc.
The door has three home made hinge bolts. Woodscrews with the heads turned off, screwed into the door with a drill chuck, and the post marked by partially closing the door and then drilling clearance holes on the the marks on the frame.
A thermostatically controlled 2Kw fan heater keeps it cosy, and rarely runs, once up to temperature.
A 60 Watt tubular heater, beneath the Fitting Bench (18" deep x 8' long) is left switched on when frost is expected.
Rust has not been a problem, so far.
Benches are all steel topped, on 1.5" angle iron frame. 2.5" angle iron load spreaders under the benches for 300Kg each, Mill/Drill and Lathe.
4" Record vice mounted on Fitting Bench, so that long work can protrude through the door.
Depending on your viewpoint, Compact or Congested; has some commonality with Rubik's Cube!
Power supply is a ring main of eleven metal clad twin 13 Amp sockets, fed through a RCD. (The feed to the VFD is via a suppressed socket)
Lighting
Two 5 foot individually switched fluorescents on the ceiling. The lathe has a 24 Volt 50 Watt Halogen light that came with it.
A single LED in an ex industry worklight over the Fitting Bench (Sadly also used for storage), and similar on each side of the Mill/Drill.
If the mains drops out for any reason, a battery powered emergency standby lamp on the end wall provides enough light to make a safe exit.
A handheld, LED torch for when my old eyes cannot see what has fallen on the floor!
Walls, door, and ceiling, undercoated from the benches upwards, with white emulsion paint, followed by white gloss, to brighten things a little.
Some of this may help, I hope.
Howard