1990 4 Runner

I just bought a 90 4 Runner it has just about 200,000 miles on it. it runs great except at a stop sign the idle somtimes drops to 500rpms and sounds bad but once i get driving its fine. Even though at timesi t seems to be lacking power. Is there anything I should check or replace on this. I have never owned a 4 Runner not even somthing with fuel injection so I am not sure what to do. Before I was dirving a 77 camaro...v8350. so its very differant ... Also at 200,000 miles should I replace the engine? or just rebuild it or what. The auto part store said the engines are about 2200 and up thats ALOT .. are there any places to order cheaper engines .??..........