We built 6 yrs ago and did put stairs from the garage into the basement because I didn't want to always have to access the basement from the house. Has turned out to be a great addition. The wife fought it at first but now that we are in she is very happy. We have a second set that do come into the living area. Also do a big mud room off the garage. Ours is about 10 x 10 with a large closet and hooks to hang jackets/coats. Very handy. We did an open plan too but it is a bit different than what you have. Don't know if I can post the layout but will try. We are on a flat lot so it is a single story with a full basement. I also had a lift in the garage (moved it into a new shop with taller ceiling) and with the 11'7" garage ceiling it was okay for a car but you couldn't raise a full size truck all the way up. I would make sure you have at least 13' high ceiling where the lift will be. I don't know the cost for a precast floor but 80 grand seems high for the garage. I have about 100 in a 66 x 80 Morton with heated floor, Mezzanine, spray foam walls, steel ceiling, lighting, electrical and all. The lift was only 1500 new.