Since it sounds like you've worked on them before and know what common failures are and the solutions, why not?
Stay away from the S4 - the B5 generation is notoriously unreliable with the 2.7 TT V6 and even the enthusiast owners won't call this a reliable car. Low mileage or not, I can not recommend that generation of S4. And that's before getiing to what a pain in the ass it is to service - this thing is a mechanic's nightmare.
Do some reading and you'll see what I mean. From the same era the E46 M3 for example while not a Subaru would be a much better choice when talking about that type of a car.
Boxters are around $22k, but I can't find a manual one. Plus, I don't want to see how much a Porsche service is in Costa Rica.
Possibly only the dealer has the tools/experience/know how to service them. Parts must be a nightmare as well.