Wanting to upgrade an Amish building

LINK

I ordered this building from the Amish builders a couple of years ago. It's mainly been a catch all for all my unnecessary crap.
I've decided to turn it into a workout/man cave area. I want to start with the floor before I do anything else. I'm guessing this is just a sub-floor? Would I add 1/2 plywood over top of this?

Also, I want to wire it for electricity, as I have a secondary power pole at the house (about 20 feet away).

Last, I want to insulate it. My questions for this board:

1.) 1/2 inch or 3/4 plywood for the floor? Non-OSB board?

2.) Can I find someone to wire it, with a breaker box, without a permit? I have the power pole 20 feet away; I was wondering if it can be tied into that without permits and such. Power pole was used for air compressors and welding machine in the past.

3.) Insulation. Would rolled insulation work, insulation board, or spray foam? Or any combination of those?

Any input would be appreciated.