The Boxster is a great car and yes it is a purpose built convertible! I bought it because I wanted a fun car with top down driving. This car has exceeded my expectations. Find a well cared for car and your maintence costs should be lower. Do some of the maintenance your self and your costs will be reasonable. These aren't hard cars to work on, if I can do it so can you. Lots of valuable information on this forum as well as other resources. Find a good indie mechanic to handle the things you don't want to do.
This car is a giant step up from a Mustang IMO. My son had a 2002 Mustang and there is just no comparison to my 2001. My sons Mustang had way more maintenance issues than my Boxster. These cars are a great value. Yes, I put in Premium, which in reality is not that much more than mid or regular. If you are worried about that cost, the Boxster may not be for you at this time. I also drive it aggressively so my gas mileage is not exactly stellar, but I am good with that
Seriously, for what you get for your money with these cars, you can afford some maintenance and premium gas.