On your HVAC. Here's one suggestion. What ever you put in, you have to take out ! Meaning whether cooling or heat you need to allow for exiisting "air" to circulate. Much like building a cowling for one of our models. heat being generated by the engine, and allow "heated" air around the engine to escape to the outside. Likewise using your cars airconditoning/heater appication, you "crack" a window to rapidly exhaust the heated air out quickly to allow the cool conditioned air work in your "HOT" car if your using the "cooling" side of the HVAC.

Also, IF you go the existing HVAC system make sure it is big enough (tonnage wise) to support the additional square footage of the shop, without laboring the original system. Good point on your "return" air application. Most likely you would have to "crack" a window, or let the "conditioned" air/heat seep our through the various areas, doors, windows, garage doors, etc.

It sounds like with what info you've given you may be better off to have a seperate wall mounted A/C / heating system installed. Maybe one like the motels use, aka... the window mounted HVAC unit, where by in one swoop you have ac, heating and fresh/return air.
Your only concern is, can your house elecrical panel support the additional amps, where the panel is located, and where you want to run the wiring, etc.

Just some ideas for you to ponder.