....
matter. A 30 X 30 shop in a 30 X 50 building sounds like a good start
that I could change as needed.

I'm wondering if I should have a company do it all or be my own contractor
and form out concrete and building separate?

In this part of the country you'd hire the building separate from the
concrete, no question. Up here (MN) they run construction specials in late
fall. Everybody trying to get one more job before winter shutdown.

Be sure and plan for doubling the size of your building. Ain't no such thing
as a shop or barn that's big enough.

Karl