Good effort! Did mine last year, similar size, similar cabin type structure. I went with a rubber roof to help keep the damp out and had the roof insulated during build. Also insulated walls and floors with celotex and put in an extra ply layer in the floor to cope with 200kg machines being dragged about. Had walls and ceiling plastered for good measure. Last was the flooring, carpet tiles which are cheap, hard wearing, look good and easy to fit or replace.
The whole thing is heated using a single panel heater with a thermostat and it's been toasty warm all winter. If it wasn't full of broken cabinets I could sleep in it!
One thing I regret was placement of switches - put them high on the wall so you can reach without pulling out the machines! I'm have to work around this now with remote control plugs as I had them placed near the floor.
Anyway good lick, look forward to seeing what you end up with, and how you fill it.