5700 sq ft to heat....you already said you have a good insulation plan.
Looks like I saw insulation board under the slab??
How is the rest of the floor plumbing laid out? Assuming it's runs are kept as short as possible with a 120' long building, with 5/8" PEX in the floor, I don't know why a ~200K BTU boiler wouldn't be enough.
I am heating 20K sq foot on a 375K, half the slab is not insulated underneath, and the only times I need a bit more boiler is when it's 0* for days on end with zero sunshine.
http://www.wellertech.com is the plumber I use. I'd be glad to give them a call and see if they'd be willing to chat with you and help you make sure your local guy is up to par.
FYI, my old shop is 60x80x16, uninsulated slab, and we were heating it with an inefficient 200K used oil boiler. No problems.
Chris