m3bavaria,
Let me preface this by saying I don't own a Gen III but I've been involved in the Mitsu off road community for years I'll answer your question based on my experience so far... and "I did stay in a Holiday Inn Express last night".:sombrero:
For options... it depends on what you're looking for and where you're located.
If you're in the states, ALL montero's are old because Mitsubishi stopped importing them a few years ago. IIRC, the US current body style is already a generation older than the most current body style overseas.
As for options, there are plenty depending on how 'hard core' off road you want to go. For basic expedition wheeling, the stock trim is very capable.
The biggest limitation (from an options POV) is the available drivetrain options.
GEARING OPTIONS: NOBODY that I know of makes gears for the Gen III or newer Montero's other than Mitsubishi and their selection is pretty slim (nothing over 4.90:1 on the R&P's & no transfer case options whatsoever).
I know you're looking at the current body style (gen III) but in case a gen II (92-2000) might be in the cards.... you can upgrade your 4.62:1 transfer case gears to 3.15:1 through a manufacturer in Australia. There is also a standard Mitsubishi rear differential Lock (94-96 SR, 97-2000 w/ the Winter Package). You can also upgrade the differential ring and pinion gears from the stock 4.2:1 to Mitsubishi R&P's that are 4.9:1. Doing both gear swaps will allow you to run 33's w/ an 'effective' stock gear ratio off road. Alternatively you can run 35's with just the slightest amount of power loss and 37's w/ just a bit more power loss (like running 35's with the original stock gears) off road. On road, the R&P's will work in your favor but the xcase gear will be the stock gearing.
I currently run 35's with the original gears. I notice a slight power loss but then again, I'm not like alot of owners that drive it expecting it to behave (accelerate, corner, etc) like a car.
LOCKERS: ARB is either offering or working on a rear locking differential for the Gen III. Mitsubishi may offer one but I don't believe it ever made it to the US & getting options from other countries through Mitsu Motors USA is an excercise in futility. ARB does offer one for the Gen II's big front diff currently but not for the Gen II small front diff.
ARMOR: JOAS may offer a moderately tougher skid plate option but for hard core armor 1/8-1/4" thick steel Skid Plates, & Rocker Panel Protection, etc you are looking at built to order solution from your local neighborhood fabricator.
CAMPING OPTIONS: Most options like RTT's, Roof Racks (std brands), Bumpers (ARB, TJM, ECB, etc.), etc are available from a variety of manufacturers.
Other more cosmetic options abound so long as you're not hoping for functional off road use (i.e. brush guards, light guards, etc.)
HTH.