Call your insurance company first. That huge jump in premium alone will probably make up your mind for you.
Also, dial up the local Porsche dealer. Ask them how much a brake job is on a Boxster (not "Boxer"). Then call the Honda dealer and ask the same on a Civic/Accord. Ask the Porsche dealer how much it would be to replace a head gasket. If it still sounds inexpensive to you, then go for it.
I wouldn't touch a 10-year old Boxster with 150K miles. And if you plan on trying to get a loan for one, it'll be VERY difficult.