So that being an '05 ECSB Z71, it could be an L33 truck. Get a picture of the RPO sheet if you can. L33 was the Hi-Po 5.3 with an aluminum block, flat top pistons, 799 heads and hotter camshaft. Made like 315hp instead of the regular LM7's 295hp.
What's been mentioned above is mainly just maintenance stuff that can/will happen with any car. Wheel bearings don't go out any sooner than anything else (my current truck has 187k on it and the entire front end is 100% original). Engine gaskets are also considered a maintenance thing at this age. Every seal/gasket under my truck needs to be replaced at this point but I expected that/knew it when I bought it. Engine is coming out for a reseal soon, trans was rebuild just before I bought it, T-case needs a reseal and axles are getting done now.
Gauge cluster stepper motors like to go out, so if the oil pressure gauge is pegged or moving around erratically, fuel gauge doesn't read, speedo is stuck in a spot, etc, get the gauge cluster rebuilt. That runs ~$175 here for a basic rebuild, there's also guys online that can customize them any way you like. Steering wheel buttons died/stop lighting up. Can find them online easily.
Transmissions are the same as other maintenance items, just expect a rebuild at some point. That will have the 4L65E in it, which is slightly stronger/upgraded from the 4L60E. Depending on mileage, it's likely been rebuilt already or will need to be at some point in the future. They're perfectly fine behind a stock 5.3, don't buy into the "60E's are junk transmissions" online BS.
Rear axle is a 8.6 10-bolt, basically an evolution of what's in your square. Plenty durable for a half ton truck.
Front axle is an AAM 8.25 IFS differential. You'll likely have 3.42 gears (GU6 code - the base, most common gear in '05-up Z71s) but 3.73 (GT4) or 4.10 (GT5) was optional. If it's a bench seat truck, it could have either the push button 4wd on the dash or the manual lever in the floor. If it's a factory bucket/console truck (easy way to tell is if it has arm rests, that's factory. If it doesn't have arm rests, the console was added), it'll have push button 4wd. The push button will have an Auto4wd option, don't use this. Ever. It's really hard on the transfer case and bearings. Just use 4hi/4low as needed (same as your square, only in slippery/offroad conditions).
Bose was an option and is a pretty darn good sound system for a stock setup. Mine works perfectly fine, even at 20 years old. There's a tiny little 5.5" sub in the console, tweeters on the a-pillars, 6.5" round speakers in the front doors and 4x6"s in the rear doors.
The transfer cases are known to have "pump rub" in higher mileage trucks. If it gets bad enough, it'll rub through the case and you'll lose all your fluid/burn up the T-case. Same as a trans, eventually it'll need a rebuild and a "pump rub kit" is a good thing to throw in while you're at it. Not a hard job.
Another maintenance thing, check the u-joints. These have big aluminum driveshafts (5in diameter IIRC), you don't want that getting slung out.
Extended cabs tend to not seal as well as crewcabs at this age. My last one needed all the door seals and door pins/bushings. All the seals are available from Precision (about $600 for all of them) and the pins/bushings are available from several different places. Make sure to check the rear door hinges closely. If they don't shut well/are badly misaligned/are tearing out of the door, new hinges are like $175 A PIECE! I just recently found this out.
But realistically, what these do need at 20yrs old/200k+ miles is simple and cheap compared to most other vehicles. These are the most comfortable/reliable vehicles GM ever made IMO, there's a reason so many are still on the road and why I keep coming back to them. I'll never be without one from here forward
