I didn't receive the original post, so I'll answer both.
In the last four years I've bought two houses. Home inspections
cost 350-400 (in Silicon valley). Both times the inspectors
seemed competent but did not find every last detail... they're
looking for the large thing that will cost you $$ soon.
I would pay the extra for getting the shop inspected, especially if
you are in an area where it's customary (or legally required) for
the seller to pay for fixes before the sale closes. A cracked
or sagging foundation might not be apparent to you but the inspector
should find it.
A 30x150 shop? What did they do there, build missiles?
Eric Murray www.lne.com/~ericm ericm at the site lne.com PGP keyid:E03F65E5