Any vehicle with 150k+ will likely need several hundred dollars in maintenance. It could probably be driven without doing the work, but that will vary from car to car and how anal you are with keeping your vehicle in tip-top shape. Shocks, bushings, tires, brakes, fluids, etc can all add up really fast and hit $500+ before you know it.
Mitsubishi's engines tend to need valve stem seals between 150-200k miles. Not the hardest job in the world; can be done DIY-style in a weekend by a shadetree mechanic and a shop manual. The bottom end is more important to consider in the used car purchase as it's costlier to rebuild. Depending on the smog laws in your area, you could just live with them for a while. It won't hurt anything. Run some additives to recondition the rubber and keep an eye on oil consumption.
Don't shy away from the 1st Gen Monteros. My '89 LS was a great truck and it went everywhere my Jeep and Range Rover buddies said it couldn't. And the difference in engine power wasn't apparent when crawling around in the dirt or on the rocks. Remember, the SR's rear axle can be bolted on to virtually *ANY* Montero pretty easily, Gen1 or Gen2, so don't let that sway you should you find a clean car without a rear locker. You can also pick up Eaton lockers for the rear for $210, or an Aussie for the front for $290.