I am in the process of finding my first 4runner right now. From my experience, I would take it in to a Toyota dealership to have them do a full inspection, espcially with no service history available on it. It is worth the investment. I was looking at a beautiful 99 limited with 155k mi on it. I had my boyfriend, a dealership mechanic, look under the hood and found lots of leaks...bad news. He also found aftermarket stuff and said it looked like it wasnt maintained very well. It was so clean on the outside and I was ready to buy, but glad to have found that out and not gotten into something with problems.
The dealership can also give you estimates for fixes that you can use to negotiate price.
I'm staying patient and waiting for the right one to come along, even though it is very hard. I think if any 4runner is well cared for, they are more than likely to be a solid vehicle with few problems...its just that the people who love and care for their 4runner dont want to give it up!
Best of luck to you.