You'll find a rare few stockers. You'll most likely get into it yourself; when I got my truck I swore up and down it was staying stock. You'll be amazed at some of the things this engine's capable of.
Most owners follow the factory manual for most maintenence, and even do some tasks more frequently. There's a few really good guides in the do-it-yourself section, and in the maintenence section; look at the "Stickys" at the top of the forums.
There're no serious issues with the motor and tranny in general, but each model has its quirks. I'm by no means an expert, but if you look around the forums you'll see a few issues come up. In the MAJORITY of trucks, you'll see no problems, but you will have the occasional one that does; that said, the first series motor ('01-'04.5, LB7 engine code) has had some issues with injector failure; but most of the time it's covered at no cost by warranty- '01-'03s have a warranty extension on the injectors to 7yrs/200,000mi. On the second series motor ('04.5-'05, LL7 engine code), a small percentage have an issue with overheating; it's easily fixed in most cases, again, often under warranty. The third generation, ('05-'06, LBZ engine code) is not known to have any "common" problems yet.
You won't have transmission issues unless you power up your truck and beat on it. Thats not to say you WON'T have tranny problems if you keep it stock, but how you treat it is really the biggest factor on life and longevity. You'll see many trucks go to 200,000...250,000 miles without needing tranny work. Changing the fluid and spin-on filter is crucial to tranny life and performance.
That said, you'll find that getting parts such as tranny spin-on filters, fuel filters, oil filters, etc is much cheaper and easier if you get them from vendors on here. Look around the forums and you'll see who people are buying from and who people are reccomending.
Here're some of my PERSONAL reccomendations; this is by no means a complete list or anything "official":
SoCal Diesel, owned by Guy Tripp- Very helpful guy, great deals on EFILive tuning software, custom switch panels and OEM integration, lots of performance engine parts, and other upgrades including Billstein shocks (you'll want to consider these regardless of milage or usage; MANY times better than stock shocks, for a great price)
http://www.dieselplace.com/forum/vendor_redirect.php?id=32
Fleet Filter, contact Brian Schneider- Great company for filters of all types, high quality filters at prices MUCH less than what you'll pay at a dealer...
http://www.dieselplace.com/forum/vendor_redirect.php?id=53
Merchant Automotive, owned by Eric Merchant- Great guy, knows anything you'd want to know about your truck, and happy to help out. On his site you'll find almost anything you need for your truck, OEM parts as well as performance parts, along with a few products he's custom designed for OEM integration.
http://www.dieselplace.com/forum/vendor_redirect.php?id=21
Again this is in no way a complete or official list, just a few vendors I personally reccomend. If you need any OEM parts for your truck, a great site for that is GMPartsDirect;
www.gmpartsdirect.com . Their prices are much better than dealer list, and they have most every OEM part available.
As for milage, you'll see trucks on here from 0 miles brand new all the way up to a few hundred thousand miles and still running. My personal truck just rolled over 100,000 miles, and I've got no complaints at all.
