I would definitely go with insulated doors. I looked to see what mine are and was surprised that the R value is just over 18. Same type door on my shop building. If you use an opener it should be adjusted to compress the bottom seal when closed. This has kept mice out well for me. My shop is well insulated. I also have a 12" wide by 14' tall sliding door. It's got the pink R-10 foamular insulation lining it with a covering of luan underlayment over laying it. My shop is over 1600 square feet and at the moment I am keeping it at roughly 60 degrees with three 1500 watt heaters set at 60 degrees. It was 12 degrees this morning but comfortable in the shop. This is my temporary heat because I haven't got the floor heat operational yet. The point is though and it's already been mentioned is you won't regret insulation even if you didn't heat it. In the summer it takes days for it to become even 80 degrees in the shop when it's in the 90's outside. If it were just a shell I would feel and experience large temperature swings. Hopefully this year I will get a mini-split installed for summertime comfort.