We have a saying about used high line MB products. RUNAWAYFAST! But thats just from a repair and maintain perspective.

M3

1 word PORSCHE but not the boxter. I've had several and kept my '88 911 turbo. Lowered and 450 ponies. It'll smoke the 3 candidates that you listed. I'm sure that you can put your hands on a real porsche for under 50K. Happy hunting.

In my opinion the SL5 is the only way to go, but the M3 is also a nice choice. The Boxter is a chic car. My buddy has a new X-50 twin turbo and they gave him a boxter as a loaner. He would not even drive that POS.

M3!!!
If you go Porsche, don't get the Boxter.
PLEASE don't get the Boxter

viper gts really fun car

M3 Coupe - any doubts, just read the reviews in ANY of the major mags.
I drive a 540i, and my wife drives a 323ci. They are just GREAT cars. An M3 would be 'multiple O's'.

The M3 will give you: (a.) speed + handling (b.) looks (c.) comfort (d.) luxury (e.) practicality - seating for four, and a full-size trunk (f.) technology - navigation, etc.

M3 M3 M3 M3 You will never be dissatisfied I own a 97 model plan on driving till the wheels fall off!!!

I have driven all three and own the Boxster.

Depends on what you want to do with it. The Boxster will be the most fun to drive -- not even a close call. The SL will be the most comfortable. The BMW is nice enough car, but not my style. (I am assuming you mean the M3 coupe, and not the M3 roadster which has NO REDEEMING QUALITIES.)