20k km is not that far, so chances of anything major going wrong with a given engine are not too great. On the other hand, your mechanic is right: there are lots of potential points of failure in an engine with that many miles on it, and there's no way to check them all out or do preventative maintenance--not for $5000, not for any amount. Even a complete engine rebuild will leave transmission and rest of drivetrain, suspension, steering, electrical and electronic systems, cooling, etc. etc. etc. vulnerable.

You'll decide about the Suburu, probably based on whether you can find parts and have repairs done in the places you're going. You probably already know that Foresters are pretty reliable, run for long distances, and manage difficult conditions well. Again, 20,000 km is not a long distance in the general scheme of things.

If I were you I'd be questioning myself closely about the need to limit myself to manual transmissions. If you're determined to bring a vehicle from North America, you've eliminated virtually all the possibilities before even getting started. That had better be pretty important to you, because it's going to have a huge effect on everything--preparation, budgeting, levels of anxiety during the trip itself, resale value, and more. I might even think about buying a vehicle in Europe, where manual transmissions are far more common (but I don't know much about how that might work, so take with a grain of salt).

Hope that's helpful.

Mark