I just did a 28 x32 last week and I bought the R13 bats for the walls for $260 from menards and the ceiling I had a company blow in the for $600. So for $860 I had a fully insulated shop. Just save a few paychecks to do that before you buy the heater. I feel like 6 months of running the furnace would pay that money back in savings.
But regardless I think an 80,000BTU ceiling hanger Mr. heater from menards for $500 would do it. I have the 50,000BTU In mine that's about $400. since i'm insulated it will cover 1,250sqft and i'm only 900.