As others have mentioned, thenewx is probably your best source for 2nd generation Xterra technical knowledge and troubleshooting. I can't speak specifically to the timing chain guide issue, as my model year wasn't afflicted by it, but I do know it's a regular topic of discussion over there.
Outside of the known issues (SMOD, timing chains), this platform is robust and reliable. Obviously "wear" items will need to be addressed, but if the truck you're looking at has been maintained you shouldn't have any other major issues. There's plenty of these on the road (including Xterras that have seen a lifetime of off-road use/abuse) with 200k+ miles. There's even one over on thenewx.org (an '05 I think) with 300k+ miles with little more than regular and routine maintenance.
The aftermarket offerings, while paling in comparison to the Jeep and Toyota crowd, is strong enough to provide you with anything you will or might need. There's several manufacturers of bumpers, sliders, skids, roof racks, and other exterior protection and accessories. Although there is only one snorkel manufacturer for the 2nd gen Xs.
There's also several options that exist for suspension upgrades that currently exist, ranging from simple spacer/shackle lifts to full extended travel coilover and leaf spring upgrades. For the most technical off-road applications, you can even go as far as "Titan Swapping" the Xterra. This involves taking control arms, extended tie rods, CVs, and coilovers from a Nissan Titan and bolting them directly to your frame/differential. This gives you the track width and travel of a full sized truck on a mid size SUV, and is essentially comparable to a mid-travel IFS setup available for most Toyota platforms. Even the stronger M205 front differential from a Titan bolts right up to a 2nd gen Xterra. Again, all of this isn't necessary (depending on your application or wants), but it is an existing option.
If you're just looking for a reliable, seasoned SUV with an good trail chops and a unique look, the stock xterra (especially in the offroad trim with the rear locker and M226 rear diff) is plenty capable out of the box and will get you most places with nothing more than a good set of A/T tires.
I hope this helps.