(well, OP, you just indicated you passed on this one, but if you're still looking for one... I spent a bit of time writing up the stuff below, and it'll hopefully benefit someone, if not yourself...)
I own one of these (mine's a LWB 'XL', and so is yours, if it came stock with the 5.3L, as that was a LWB-only option for '03)
Things to consider (or check for, besides what's already posted above)...
- Are you handy working with vehicles? If not, this truck will be a lot of trouble & $$$$, taking it to the garage all of the time. But once you've caught it up on its maintenance, it'll be pretty reliable. Parts that you source yourself are fairly inexpensive, and still available.
- Be prepared to put a full front suspension on it, if the prior owner didn't. Upper ball joints are especially problematic, so get quality replacements for those, at a minimum. Or budget in new ones every couple of years.
- If it has the air suspension in the rear, and it's working now... it won't be. Parts are getting difficult to come by. Thankfully, it's fairly easy to convert it to coil springs / shocks.
- The transmissions tend to go south on these (4.2L as well as the V8) as you get closer to 180-200K miles. Putting in a corrective shift kit can prolong the life of the 4L60E substantially. We have lots of threads on this.
- The fan clutch on these is electro-viscous, and goes out all the time. You can do a conversion to a thermal fan clutch (GM went to this for the '08-'09 models; even they got tired of replacing them). Besides the parts, you need to have someone remove the fan codes from the PCM (we have resources).
- HVAC can be problematic because the airflow out of the upper / lower dash & defrosters, as well as the temp of that air (for both driver / passenger, since it has dual climate) -- is controlled by actuators, which break. That's not terrible, but reaching some of them in their various behind the dash locations, is. Again, we have info on replacing them (and there's no vendor making 'bulletproof' ones, either, sorry to say).
- The 5.3L is a great engine (made even better with a tune - see: fan clutch, above). The 4.2L is, too. But the downside of the 5.3L in this truck is that it's shoehorned in. Things are tight in the engine bay, with the V8. One great thing about the LM4 is that there's no cylinder deactivation (AFM, DOD) to worry about / remove (those engines use special lifters (among other things) in half of the cylinders, and the early ones tended to have problems).
- With the salt air in FL, you'll want to look for rust. On the body, the bad areas are the bottom corners of the liftgate (it tends to rust from the inside -- pull the lower trim to find it), as well as the front doors (again, at the inside seams -- open the door and look downward.) Northern trucks require a careful overview of both the frame & suspension attachment points (rear control arms are especially susceptible). Check for rust thru on the brake lines, as well. One more area on these to watch out for is the fuel filler neck / tube. Easily replaced, if needed.
- Wanting to put a Bluetooth-enabled radio replacement in? Get an aftermarket harness. Trust us, and do NOT cut your factory plugs off to wire up the replacement. Also, if the vehicle has the Bose upgrade, the amp goes out with regularity, and so will a replacement, if you buy a used one from eBay, etc. Again, we have lots of info on upgrading the sound system. If it has the factory-optioned rear entertainment DVD player with the drop-down screen (from Panasonic), and it's working now -- don't have your rear seat passengers get too attached to it.
Finally, fit & finish on these things was typical early 2000's GM -- which is to say, a bit lacking, compared to what you see in new cars today. You'll have interior squeaks / rattles that will pop their heads up; the shift lever handle will constantly loosen up (until you wise up and put some blue loctite on the small allen screw that holds it to the lever), and if you like to lean on the storage console lid, you'll hear the results of stress fatigue on it (if it doesn't break off outright). The switchgear (especially in the HVAC control head, and the radio) is painted on, and with repeated use, will flake off, leaving unsightly white patches behind. By that time, you'll have remembered the functions that used to be displayed on them, so you'll be fine.
On the exterior, the gap between the front bumper cover (fascia) and the grille / fenders, is, to be frank, unsightly in a modern vehicle. If it bugs you, the Denali versions improve things substantially. Oh, and since GM never updated the sheet metal, you have all MY from '03-'06 for parts for the LWB, and if you need hood / fenders / doors, you can use them from any Envoy (even the short version).
If the one you're looking at has the heated leather seats, you'll find the bun warmers are really nice, and you can set 'backrest only' on them, as well. I do love mine. When the A/C is working properly, you can use the truck as a chest freezer, if you're one that stocks up on provisions. I wind up turning the fan speed down after a few minutes. Same for the heated seats - after a few minutes, I have to drop down to 'medium', especially on the seat portion. And the '03-'04 had adjustable upper *and* lower power lumbar, in the SLT. Really nice to have that kind of adjustability.
That's about all I can think of. I've put a fair bit of $$$ and time into mine, catching it up on maintenance, etc., since I got it about 5yrs ago - but it's now a well-running truck that I wouldn't hesitate to take anywhere. Good-looking for its age, too (e.g.; body damage, rust).
