First off, get ready for a LOT of bashing on the 6.5TD. I have owned a couple of them, used them pretty hard and had good service out of them. My current truck is a 97 K3500 crew cab dually.
The 6.5TD is not a bad motor, but it does have some quirks. I have heard horror stories about all of them, Dodge, Chevy and Ford. Basically I look at it this way, people generally won't talk about the good service but will shout the shortcomings from a mountain. (How many times have you called your State Rep just tell him/her how good a job he/she is doing?).
The 99 Sub should have all the improvements made to the 6.5TD. Is it 1/2 or 3/4 ton? I *think* since it is a suburban it will have the "S" code engine which had more emissions controls than the "F" code engine. Not necessarily a bad thing, just a couple more sensors/electronics.
Biggest thing w/ these engines is the "drive by wire" GM introduced in '94. The electronic pump control is mounted on the pump, in the engine V and below the plastic "6.5 Turbo Diesel" cover. It soaks up a lot of heat and fails. GM usually wants to replace the whole pump, but several companies offer an aftermarket part to replace just the controller (and they relocate away from the pump). Usually there is nothing mechanically wrong w/ the pump, just the electronics. Since it is a diesel, it will probably have 6 or 8 lug wheels regardless of it weight class. The suspension will be heavier due to the weight of the diesel.
What I like about these trucks is that enough people bad mouth them and make them extremely cheap to pick up. Will they run w/ a 5.9 Cummins or a 7.3 Powerstroke...no. But they can be dependable, cheap to own vehicles and do some moderate towing. i regularly move 12 5x6 bales of hay w/ mine and it hasn't snapped in half yet.
If you get the truck, check out SS Diesel Supply and Heath Diesel Power. They have a lot of aftermarket, improved parts for these engines.
Sorry to drag on, but I happen to like these engines and think they get a bum rap a lot of the time. That and I don't want to pay 40k for a new Duramax or (shudder!) a Dodge.