Just last summer I put up the same building you are talking about doing. 30x40x12.5 ceiling. I used Cleary Building company. They are nation wide and was a one stop shop.
I chose to only have the shed put up, outside finished, with concrete floors. I will run my own electricity at some point, but I did have doors with openers installed. ( so far 125ft drop cord works )
This is the best pic I have at the moment...
Currently I just use it as overflow, park the toys in it. C10,SUB, Blazer, 94 Silverado, antique one wheel camper, 317 deere mower, 3 honda quads....etc etc...
House garage is 28 deep and 36 wide and I still work on things in the attached house garage.