I like the GMT800 stuff. Even though they are getting to be 20+ years old, they are pretty reliable in my opinion and experience. My son and I have dug into them a little bit and have been able to fix any problems so far.
I have an '01 K2500, 6.0, 4L80E, 4wd that I bought with 279K on it. We did a a bunch of work to it because we could and I got the truck for cheap. That gave us some good experience digging into these. The only thing that we didn't do, was pull the cab off of the frame, or go into the 14FF rear or front diff. Had the whole interior out of it, too. Slightly crazy, by some perspectives. But I enjoy the truck now.
The manual transmission is a rarity in 1/2 tons around here. But I am not a hater of the 4L60E like so many others. I have seen so many of them abused with dump bed trailers, no maintenance and neglect, that I understand why they die.
I watched a couple of them go 160K and 240K of driving around town in company trucks, and an '06 Ferd need both the transmission and T case replaced before 100K.
I bought an '01 GMC from my work that had over 300K on it. Original 4.8 engine and 2nd transmission. The last guy who drove it, used to drive with his left foot over the brake because I could follow him and the brake lights were almost always on. The TCC won't lock when the brakes are on. He would also haul hay on it for his horses..
Those trucks seem pretty easy to work on in my experience. ANYTHING to do with the doors and windows can be done much easier than the GMT400 trucks.
The most difficult task in one is replacing the heater core. It just takes time. There are YT videos that show what to do. We have pulled a complete HVAC system apart in one, just to see how it goes, in order to be prepared whenever my son's '03 Dmax needs it.
A 4x4 truck will need a pump rub kit in the T-case from Merchant Automotive, if you don't find a 2wd.
There are other things that could need addressed, but you may already know a bunch of them.
