I personally would avoid trying to paint in an attached garage, especially if you're using paint with hardener which typically contains cyanide compounds. The other issue is dust that probably exists in you garage from doing any metal work and sanding bondo, which is going to get in the paint. It probably isn't a big deal for primer coats, but you definitely don't want dust in your finish coats. I know that everything in my garage is coated with dust.
You also need to be concerned with temperature control. Winter's here for me and painting season is over, except maybe for a little primer work here and there in the garage. I don't know how cold it's getting in your area, but a paint booth will probably need heat as well as ventilation.
As far as assembling first, I plan on painting major components separately.
I'm sure others will weigh in with their opinions.
