[demick]

None of the financial reasoning I'm reading here makes any sense.

It sounds more like you don't want to keep the Boxster and are looking for something else. If that's the case, by all means, get rid of the Boxster.

But if you really do like the car and would like to keep it, but are simply afraid of possible future costs, do this instead: Take the $25K that you would otherwise be out-of-pocket on your Boxster-Evo swap and put it in the bank. Anytime you have an unexpected expense on the Boxster, take it out of that account.

Chance are, 5 years from now, you still have more than $20K in that account. Even if you are very unlucky (total engine failure), you will still have $10K left in that account. Financially, there is almost no possible way for you to come out ahead by buying the Evo.