I'm just outside of Springfield, MO and have a 30'x40'x12' that went up 3 years ago. Your layout is very similar to mine, but I ended up going the pole barn direction at the end, simply because of issues getting a contractor committed. If you are going to work on cars, you'll really like loading on the 40' side. You are also right on track on costs compared to what I saw for stick frame. Missouri, especially south, is really reasonable for this type of construction.
I would not get too worried about the truss spacing, but it is harder to hang and insulate the ceiling on the 4' spacing. It can be done though, as I did mine myself. You just have to plan it out. Do it now if you think you want it and can swing the bucks, as it would be really tough if the shop is not completely empty.
I have a 2 post lift, and my clear height is actually 12' 1 1/2". The lift touches the ceiling slightly in one spot. It's TIGHT, and did limit my lift choices. There are some that will fit in under 12' though. DMS equipment in Nixa is a decent local supplier if you decide to chase a lift and want local support. Really good guys to work with.
If you get up Springfield way and want to see the doppelganger to your shop all done and in use, drop me a PM.
