AC/Heat Unit for Shop

Hey Folks, I'm building a private workshop for use in doing my own occasional automotive, woodworking, etc type of work. Its a 40 x30 with10' walls to 16ft peak ceiling. It has been spray foam insulated to a minimum of 2" which is expected to be around 14r. The building is wood frame metal skin with concrete slab (no radiant floor heat).

I live just south of Memphis, TN in North Mississippi and looking at summer temps of 95-100 at peak and winter averaging 30-40. If its colder then that I wont be going into the workshop anyway :no:

I want to get a HVAC system but do want to deal with the Natural gas line because of where the shop is located on the property. So I was thinking of some kind of Heat Pump system preferably a outdoor package system.

I got the idea from seeing several used 10 seer heat pump package units (with heat strips) for sale on craigslist (local). This area is pretty good for heat pumps I guess because of the milder winters. And folks seem to be up grading to more efficient ones.

Any advice on size would be appreciated. Any comments would be welcomed.

Thanks