I just went through the same thing with an '06 GMC 2500HD I purchased earlier this year. It had 150k on it, decent paint, a really clean interior and ran well. Had a 6" lift, new 35" Toyo R/T's, dual King shocks, CST upper arms, but the front end was completely shot, steering was super sketchy and it needed a few other things. Here's what I did, as I always buy the best parts that I can afford, and always do my own work.
Dorman steering column bearing
new GM/AC Delco 'updated' intermediate steering shaft
2 DMax Store HD tie rods (inner/outer)
new CST upper A-arm poly bushings (CST tubular control arms)
Moog hub bearings
Cardone 'HD' CV axles
Bilstein steering stabilizer
Energy Suspension sway bar endlink bushings
flushed the power steering system & added a Derale cooler.
Night and Day difference in handling/steering after throwing some parts at it, along with a front end alignment. The G80 rear diff was on it's way out (found bearing pieces when changing the rear gear oil), so I installed a Detroit Truetrac - the truck already had 4.56's in it. It has Bilstein 5100's on the back now and they are beat, so I'm trying to justify the matching King shocks for the back and probably a set of Deaver 4" springs to get rid of the lift blocks since the truck already has 5000# Airbags on the back now (I'll still be able to tow what I want with the softer springs). Probably a rear sway bar too as my 20' enclosed car hauler catches a lot of wind on the highway, especially when passing semi trucks.
Currently running S&B intake with a 3" downpipe hooked to the stock exhaust. I will soon we putting in a transfer case (most likely Merchant Auto) as the front case on this one has a hairline crack that is leaking and it appears to have also developed the pump rub issue on the rear case as well.
I have subscribed to this thread and look forward to seeing your plans set in motion! NICE TRUCK!
2006 GMC 2500HD 4x4 (VIN 2)
6" Pro Comp Lift, CST upper arms, DMax Store tie rods, King dual shocks, Sulastic shackles, rear airbags, 4.56 gears, Detroit TT, 35/12.50 on 20x10's