If you want more room and convienance of a rear seat, than definately buy it. The only down side of the Runner is you wont get as good as gas mileage since it is heavier, not a large drop but usually a few less mpg's. With 250k on the engine, you should rebuild it, especvially if you plan on keeping it for a long time. You should be ablke to rebuild it for under $500, or under $1k if you do a mild performance rebuild. A buddy of mine rebuilt his '84(6 months before it was stolen) using enginebuilders performance parts, the gains were noticeable, he was able to keep up with me in my modded 3.0, when I used to leave him far behind climbing the Sierras. I think he only spent $600 in parts.

As for your rig, I would clean it up, i.e., fix all the little things like tears, paint chips, ripped carpet etc., if you have them and sell it. You should be able to get close to $3k for a clean good running 2nd gen Toyota here in CA. You can then use the money to build the Runner.