Looks like youve got it all figured out and just a decision on how much you want to spend (pre insulated or you doing it after the fact) I'll chime in on the concrete. Assume you are going to pour over mesh and bar with a good base gravel pack? I would probably go 6" myself due to the size of the pad to help avoid cracking. The other thing I was going to do with mine was have a section of concrete that was 10-12" thick in case I needed to set some heavy machinery or equipment on it. i.e a lathe, mill, steel, etc. With it being 64ft long, I would also go with 3-4 floor drains in the middle or 3-4 down each edge, that way you can just spray and squegie the floor when you need to. Another option would be that for something that size, I would probably build a pit as well for changing oil in vehicles or using as a storm shelter. What size service are you going to run to it? Im jealous man, I was going to build a 28x40x12 but ill have to jump through all kinds of hoops and permits due to the township and stupid property line issues. (Even though its country)