Insulating that thing after the fact will be a feat. Unless you want to rip all the siding and roofing off probably best to put plywood in all the stud bays, then fill with insulation. If they're 2x4 walls, you might could fur them out to get some thickness and r19 in there. Cap it off with metal siding to keep the weight down (plus more water resistant than Sheetrock!) since who knows if it's on much of a footing? I wouldn't worry much either way, but metal walls would just be more durable in a shop setting and be more fire resistant than plywood and less hassle to diy than Sheetrock too.
You could spray foam as is, but then if you ever need to repair or replace siding or roofing, or get a leak (and you will) you're kinda hosed.
I'd be tempted to pull all the roofing off and put down condensation blanket insulation and then put the roof back. It makes a heck of a difference and will cut way down on condensation too, especially if you start heating with anything out there. If you do heat it, you'll want to keep everything above the dew point to avoid condensation on cars and parts. It'll wreak all kinds of havoc on bare metal stuff in just a couple short years.
Insulating a smaller space within the space is probably the smartest way to do it, and depending on how you do it, could also be expandable until the whole place is done.
As far as electric, they make mud and extension rings for a reason. Would cut down on the work quite a bit.
