1. Leather, Moonroof, Monsoon stereo are popular options. If you're an off-roader you might focus your search on the optional Adventure Package. If you like the muscle of the H2, then you should get an H3-Alpha. There really isn't much decrease in fuel consumption but, the V8 has WAY more torque than the base model 5-cylinder. If you want a stick-shift...they are out there but, only with the 5-cylinder. The early 2006 models have had issues with head-gaskets. Sunroof drains are a very common problem on all H3/H3Ts. Transfer-case and front axle issues are also very common and expensive to repair.
If you want the "T", you'll be forced into a 2009 or 2010 model...they were only made for about 18 months. T's are considerably longer and somewhat handicapped on tight trails, compared to the shorter SUV chassis. If you live in an urban area that requires tight parking, this could be an issue. The T's cargo-box is quite small, and if you truly want to haul something...I would suggest buying something else. Its fine for "antiquing" or the occasional "weekend project" type stuff...but it's the wrong vehicle if you want to haul gravel, dirt, furniture, or anything large. As far as "truck-duty" is concerned, I don't even think the H3T can hang with a Canyon, Toyota, or Nissan...and certainly not a full-size. On the bright side...its probably better than a Honda Ridgeline (yikes).
2. For reasons unknown to me...aftermarket parts for Hummers are pretty much limited to goofy chrome junk. Not only are most of these parts completely useless...they're often such poor quality that you would be better off with the factory parts they swapped out. For this reason, I highly suggest you buy a stocker, and then put on your own parts. Myself and few other small-time companies make REAL parts that matter. These are parts meant to improve the reliability and off-road ability, and most of the time you won't even know they are there. On the other hand...if you gotta have six plasma screens, 18" sub-woofers, and 24" wheels...have at it! You should have no problem finding one of these rolling eye-sores (try any NBA player).
3. Prices for H3's are falling fast. It depends on the area but around here...$9,000 to $16,000 is about it. Mileage is almost always over 60K. These are very high-maintenance vehicles, and you should take this in consideration when shopping. At 100K miles...the front end is usually very tired. The nice thing is you can completely freshen it up for around $1500-$2000...good for another 100K. H3T's prices are much higher...$18,000 to $25,000. Shocking considering what it is (and more importantly...what it isn't). The T-prices are entirely based on the limited production (less than 5000), and if you want to be in the club, its going to cost you.
Happy Hunting! When you find your soul mate...post up some pics!