I have always liked the Hummers and I have a chance to buy a 2009 H3 with 59k miles from a lady that has kept it up nice and garaged. I am a fairly good mechanic and do all my maintenance and repairs on my cars, trucks, motorcycles and RV. How good are these vehicles, I'll probably want to drive it a couple times a week to work (20 miles one way) and to the mountains about 1.5 hours away fly fishing and camping.

 

are you good with OBD2 electrical theory can trace broken harnesses locate bad grounds and keep the water out of the bcm. maybe relocate that thing as where it is factory is only a matter of time it will get wet. sunroof or no sunroof.
maintain all the oils clean and contained.. big factor. might cause carbon buildup on the valves and throttle body.
keep the ignition coils dry from oil or water make sure PCV system is not impeded. then all you have to worry about is passlock security issues. maybe a wheel speed sensor on occasion or the 4x4 switch and or selector motor.
but much of this happens higher up the milage chart if not cared for. about 90k miles and up

 

I've had 2 H3s, the second with a sunroof, my truck sits outside 365/24/7 and neither have ever been wet inside. I think between the 2 of them, they might have been in the garage overnight 10-12 times since Feb 2007. If it has a sunroof, once you do the sunroof drain fix, you are good to go.

Ask yourself this simple question: How many 2005-2010 Chevy Colorados and GMC Canyons are still out there on the road with the same engine, trans and near identical drivetrains? There are people here that have over 200k miles still going. If you can do your own diagnostics and work, H3s are easy to work on and keep going. There is a kid down the street from me with a 2006 he got after his Mom paid it off, he doesn't know a tool from a taco, but his H3 is still going without issues.

The FIRST MOD to do, replace the crappy azzed OEM Battery Terminals with an quality aftermarket set like Stingers or your favorite brand. Then keep up the maintenance.

 

The first things I did when I purchased my H3 a few years ago...
Changed the OEM Battery Terminals
Changed all the fluids to Synthetic -The engine oil, Both Differentials front & rear and also Synthetic in the differential.