Great vehicles, but they still have their issues just like any other vehicle.
If its a 3.4, make sure the timing belt/water pump has been done. They usually need to be switched out every 100-120k. Its a non interference engine so if it snaps, its not a huge deal. Since you have to pull the front of the engine apart, just go ahead and do it all at one time.
If it is an automatic, pull the radiator cap and if the coolant has a pink'ish or milky look to it, run. Also, if you get an automatic model, just bypass the trans cooler in the bottom of the radiator and run it to a HD auxiliary cooler. The stock one likes to eventually fail and it mixes coolant and ATF. The result isn't fun.
Be sure to test the brakes at about all speeds. They are notorious for warping rotors. Luckily the upgrade to Gen1 Tundra brakes is a direct swap and isnt much more than replacing all the stock stuff.
Run it with all the accessories like A/C and Radio off and listen for any driveline noises. The driveshaft joints getting greased often gets overlooked, not a hard job by any means but still gives you an idea for how well it was taken care of.
I would jack the front end up and check the tie rod ends, ball joints and wheel bearings. The front ends on these 4Runners seem to last a long while if its not hammered on. This will also give you a chance to check the steering rack any leaks and wear around the bushings which is common too.
StretchASU hit the nail on the head. I've had my 01 for a little over 2 years now and it is my daily, around 225K on the clock so far and it has been a great vehicle.
what the other guys have said. I bought 1 last july. had a 1992 extended cab pickup but sold it because the kids were getting so big. I love my 4runner. has 138,000 on it now. just like any other vehicle they have there problems, but nothing real big. just regular maintenance. and that link that chrishf12 posted is a very useful link. that is a very good website with lots of knowledge.