A compression test will tell you a lot. You could have a dealer or such perform one. That should also be able to test the oil pressure. Usually there are parameters for that. These two items can tell you the overall health of the engines.
As for miles, has either one had the timing belt replaced? The rex is coming close if it has never been done (110,000 IIRC) The STI should have had one by now but who knows. I know it would cost you for some of this work, but knowing could save you many more bucks in the future.
What are they years of the two vehicles? What kind of climate have they spent their life in. Salt/winter stuff? Motors are a little easier to deal with than body work.