Very decent post, sir. I would see about moving this to the JK section, as your MPG and gearing questions are pretty specific to the 2012+ Jeep.
Your MPG w/ 4" and 35s would depend a lot on your weight. Depends what you're adding on as far as mods and what tire/rim combo you choose. I doubt you'd get 18, but you'd do better than 12 easily.
With 35s and 3.73 gears (do NOT get 3.21 if you're lifting) you may be fine. Some upgrade, some don't. There are ppl here running 35s on 3.21 and seem fine. This depends on auto/manual and if 2012+.
I only have a 2" lift w/ ATs and a soft top, but the ride is fine (coming from Ford Ranger and Explorer).
You'll make do w/o the truck bed, but only you know how much you would need a bed. Taking the rear seats out of an Unlimited yields a bit of cargo space.