Hi Combo!
Welcome to Outers! It sounds like your on the right track. My Zook is very similar to your mod 'wish list'. Here are a couple of thoughts I have for you:
*2" Spring lift - Be careful here, generally lifted springs on a Sierra will ride and flex like crap. My Zook has 2" King springs and I have had to remove leaves and stuff around with the packs to make them work. They are OK now, but I probably wouldn't recommend them (My Zook had them when I purchased it). Because many lifted springs don't seem to be any longer than stock springs, you can end up with poor shackle angles too. (This is the case with King Springs anyway). A bit of a search will reveal that OME are generally the way to go with new springs.
*Longer Shackles - if you run standard flat springs then you might want longer shackles to help fit the 31's or to give you a bit more lift... HOWEVER a 2" BL, 2"springs and longer shackles is TOO MUCH for 31's- it is unnecessary lift and your Zook will fall over!!!
*Maxxis Bighorns - a great do-it-all tyre - Go for it!
*Gearing - I run TG6.5's with the standard 3.7 ratio diffs. I wouldn't have it any other way! Mine sits on 100Kmh at about 3850rpm and it loves it... (mine is a 1.3L though, if you end up with a 1.6L you could consider the diff gears (or less reduction) I suppose..)
I cant really help you with the price because I have stopped counting! I have done EVERYTHING myself and I estimate that including the initial cost of the car, I would be up to about 5K (or maybe 6K max...) You could buy a LOT of Zook for that money if it is already built, but that's just not as much fun!
Mine is a soft-top. Air-con happens naturally ;-) . No idea what models/years have AC. Sorry.
Have a look in my Members thread (in my signature line below). It might give you some ideas on what to do (or not do).
EDIT: Also, as Santos said, don't overlook a Vitara if you want to be comfortable. I could drive my Sierra every day, but they are fairly 'agricultural' in every sense of the word!
Cheers,
Pez
91 Sierra soft-top. My Members thread: http://www.outerlimits4x4.com/viewtopic.php?f=16&t=169128