i wouldn't get the sl500 or slk55 since they are only offered with an automatic.
$65k is a lot to pay for a boxter, and it isn't incredibly fast.
cayman is awesome if you don't care about having a convertible
z4 roadster is great, but for me personally i wasn't comfortable in it because i have to push my seat up so far that the door is like 3 feet behind me so i can't get in and out without moving the seat. it's also just awkward feeling. but anyways that's just because i'm short. i'd rather have the cayman since i'm not uncomfortable in it and it's a coupe so it will be more aerodynamic and faster.