I have 12 foot walls in my shop and would never want anything lower. I do a fair amount of woodworking and being able to handle the material without hitting the ceiling is huge. I've lived with 8 foot ceilings in garages and hated it. If this is something you are going to keep and use for a long time into the future, do it right, spend whatever it takes and get a higher ceiling.
How far apart are the trusses on the ceiling? OSB needs to have them no more then 24 inches or they will sag on you. If your trusses are farther apart then that, then part of the cost will be installing the lumber to support the OSB. Depending on how far apart they are will dictate what size lumber is used. It should be on edge so it doesn't sag.
I really like those pictures of the metal on the ceiling!!!! That's very nice.
4 inches is very common for driveways and garage floors. Cracking is going to be from the soil not being compacted properly, or not enough rebar being used, or none at all, and whatever is used, not being held up by chairs while the concrete is poured so it stays in the middle. Unless you are parking heavy equipment on it, six inches is a bit of overkill.
Eddie