Tankless Heaters...
I installed a small Takagi tankless water heater to heat my shop and it works very nicely and is pretty quiet. Like mentioned before, you can get tankless heaters that are designed for outside mounting, under an overhang, etc. The unit I got had an external vent inlet option and is mounted inside but combustion is sealed off from the inside air. It draws air in from the outside. The exhaust vent was made for this type of setup and draws air in around the center exhaust. Since it draws combustion air from outside I don't worry too much about vapors and fire hazards, etc in the shop - at least not with regards to the heater.
As for the PEX piping install, make sure you have it properly designed so as to work properly and get max efficiency. I downloaded a free program from SlantFin that allowed me to lay out the piping for the space I had. I got my 3 loops far shorter than is recommended but it does still work nicely. I would think that for your size shop you will want at least 3 loops of 1/2" at around 250 ft each. This is only a guestimate so make sure you ask someone who knows when you buy the PEX. Run the cold/outlet of the loop by the overhead door so that the cold side is towards the door and the hot/inlet end of the loop is more towards the interior so the BTUs will not be lost so quickly out the slab under the overhead door. Use wire mesh first and then zip tie the PEX to the mesh in the designed layout. Use plenty of rebar on top of the wire mesh and PEX combo. Make sure to insulate with foam around the perimeter between the slab and the foundation wall. 4 to 5 inch wide strips of 1" foam with a 45* bevel at the top works nicely. Before doing wallboard, don't forget to install thermostat wiring to a location AWAY from any doors and heat sources.
-Deon