I guess that would depend on what year and model you settle on.
Engine-V8 Chevy, hard to go wrong. Carb'd up thru '86, TBI thru '95, Vortec style EFI '96 on. Depending on your needs, a 305, 350, or 454 all are available and all do well, though the 305 is a bit of a dog in anything but a half-ton shortbox with no options!
Diesels are also available, but a lot of guys don't recommend GM diesels--tho alot of guys do. Seems they either run forever, or not at all. 6.2L N/A diesels came on the scene in '82 and ran thru about '93, and somewhere around there the 6.5 N/A and 6.5 TD came available. I wouldn't even consider a pre '82 diesel, they are a modified 350 Oldsmobile engine and are troublesome and hard to find parts for.
Trannys.... pick your poison. Automatics include simple and rugged TH350 and TH400's--the old three speed auto. Cheap to buy, build, and rebuild. No overdrive... which leads us into the 700R4/4L60. Early versions(82-87) had their share of problems, but by '88 or so they had them working pretty well. Can be rebuilt reasonably these days using heavyduty Corvette and aftermarket components. This tranny was superceded in around '93 or so by the 4L60e, an electronically controlled version.
Sticks... There was a Saginal three speed manual, not sure if it ever saw 4x4 duty or not. A light duty three speed plus overdrive... and the fourspeed manual Muncie SM465... basically a HD three speed with an unsyncronized bull-low first gear. '88 saw a Muncie/Getrag 5 speed (which if I'm not mistaken is currently still available as a NV3500), and the early 90's saw the introduction of a NV4500.
Transfer cases... Early 70's on trucks had three main ones, all work well for their intended purpose. The np203 was a full time 4x4 case, though many were converted with a contravertial part time kit to save fuel. Its cast iron, and sturdy, but chain drive. The NP205 is considered about the ultimate case, being cast iron and strong gear drive. The NP208 came on the scene in the early 80's as a lightweight aluminum unit, chain drive, not as strong but easier on fuel. Later trucks came with I think an NP241 (or 231? I forget which) which is similar to the 208.
Front axles... 70's trucks mainly had Dana 44's in them, though the tail end and thru the 80's up to '87 (or 91 R/V series Jimmy/Blazer/Sub) had GM Corporate 10 bolts. (1/2 and 3/4 tons). 1 ton trucks got the Dana 60, about the best axle you can buy. '88+ trucks went to the independant front end many people don't recommend for plow service or offroading.
Rear Axles... 70's 1/2 tonner trucks had mainly 12 bolt semi-floaters, 80's had 10 bolt rears. 3/4's from those era's mainly used the 10.5" 14 bolt full floater, but somewhere in the 80's they also started using the 9.5" 14 bolt semi-floater. 1 tons all had the 10.5"er, FF... considered the ultimate GM rear axle. Some 88+ 3/4 LD trucks used the 9.5" with 6 lug hubs instead of 8.
Have I forgotten anything? Or does that pretty much sum it up?
We could really mix it up and throw in S truck stuff, but we'll save that for another day... my hands are tired.