We constructed a 30x55x11 metal bldg on top of a concrete slab in south central Kansas. Wood columns, girts, trusses and purlins. REsponses below are based on our experience over the past 3 years.
"...4" open cell foam insulation in walls and ceiling with and extra 2"under the roof of the future living area..."
We had 3" of Icynene open cell sprayed directly on wall and roof metal. No venting, so we operate a dehumidifier as needed. When temps reach 100 degrees+ outside, it rarely gets over 92 degrees inside. Temperature difference between 6ft and just under the roof deck is usually about 4-6 degrees. So good call to insulate.
In early winter, ground temp under concrete slab keeps inside around 40 degree when at freezing outside. Late winter and lower temps, it's not hard to keep inside 34-38 degree with a 1500watt heater. Has to work a bit harder when we hit single digits and sub-zero. Unfortunately, we weren't up to speed on heat loss thru slab edge otherwise we would have insulated slab edge somehow.
"Just this week it has been suggested that the exterior walls need to be at least 8 inches for energy efficiency"
Is this for the living space area or the bldg overall? I think you could get by with less for bldg walls and then insulate your independent interior structure. Which is what we did for a 15x15 efficiency apartment we built inside our bldg. We built 2x4 walls and insulated conventionally with fiberglass including above the ceiling. Basically the bldg insulation creates a buffer zone for the interior room. We use an 8" cube heater which kept our room at 70 degree even when it was -5 degrees outside. We do run a dehumidifier to maintain 55-60%.
"...with 4 overhead doors in the shop area should the doors be insulated from the factory or foam be added when the wall insulation is added?"
If you spray the walls and ceilings, insulate the doors. I did our two 9x11 overhead doors using 1.5" pinkboard and foam adhesive which made a huge difference in noise suppression and wind infiltration. I wil add horizontal braces to stiffen them even more.
Considerations:
1. Insulate slab at edge to reduce heat loss/gain.
2. Find out if bldg will be built to agriculture (will leak) or residential (will leak less) standards. Take a look at Fabral's installation guide http://www.fabral.com/downloads/details-postframe.pdf
3. Paint foam with white or light colored latex paint to provide a bit more reflectivity. Use oops paint from big box stores.
4. If you will use bldg windows and doors to access the livings quarters, address how to flash well. We've had continuing problems with cheap slider windows that wind blown rain can infiltrate thru and around. We were told "Well, it is only a shed".!!!
5. If you don't vent, be sure to address humidity. I would think the building supplier would address this with you.
6. If you are in an area that doesn't require inspection, then be especially vigilent in making sure the builder gives you what you are paying for. You don't want to wake up at 2 AM because it's starting to rain and you have to get up to see if the roof or windows are leaking again!!
Good luck, Dave