This is a suzuki site, so you're more likely to get people to tell you they'll run forever... they don't. 18x,xxxis a lot of mileage. Don't buy it unless you can afford to replace inevitable breakage from wear and a re-build / engine swap anytime in the next 1-100,000 miles. I've seen them die at 200k, and some are still running with over 300k.
Parts availability is easy. Most parts stores carry stuff, and there's lots of aftermarket junk yards & fabricators. As far as Samurai vs Tracker (I've owned lots of both), each has it's pro's and con's. Pick the one that suits you:
Tracker - Better MPG, better engine, more power, more room, softer ride, cheap-to-lift to 31" tire. I wouldn't put more than a 29" tire on one, but some people love to go to 31" or larger. The reason is kicks don't have the gearing options that samurai does. If you're a 31" wheel guy, I'd look to the Samurai. For the record, I wheel my 4-door on 29" and open - and it's capable of everything but the craziest of trails. Perfect for DD / weekend warrior. Aftermarket support is getting better as the used vehicles come down in price - but it's only 1/4 as good as Samurai. If you're a 33" + guy, you're swaping to toy axles on either kick or sami, so it really doesn't matter (though, I lean towards tracker in the big-wheel dept).
Samurai - better aftermarket support (been around longer & cheaper). Used upgrade parts easier to find, and better support on boards like these. Better "cute" factor.
Sami has the advantage off-road, but tracker has it on road. Just depends on what you want to do... unless you're getting into extreme obsticles, I'd just get a kick on 29" and call it done.