As stated already, it's not at all worthwhile to hunt for the extra 50 or so horses that differentiate the base and the S models...just get the S, because you'll have been able to get one with the money you'd need to spend to get the power to the base car. The RS60 is sitting right around 300 horses, at least within 10hp of it, and depending on the options it has it may very well be there. You want to be able to hit off-ramps and straight patches of highway with some oomph, you've got it right there. From 09 forward you're definitely over 300. And considering RS60s were
starting at something like $65k new going on 5 years ago, I think that $35k-ish territory isn't really that unreasonable since it's not a regular S.
As for the wife, well, you know best how to approach that situation, so I'm not going to hand you a shovel to start digging with.
You could definitely DD the Boxster if you're ok with doing so. Are you buying this car to enjoy it or with the hopes that you can sell it later in life at some appreciated value? If it's the former, good, go get one and enjoy it, trade the BMW if you have to (but I do understand that sentimental attachment that can happen)...if it's the latter, then honestly don't get one, because Boxsters aren't going to be appreciating any time soon...you can just expect that something like an RS60 will depreciate a little slower than the rest of the lot.