Lots of choices fit that bill.  Unfortunately, most luxury brands (Mercedes, BMW, Jag, Cadillac) have embraced "planned obsolescence"; this means once they go off warranty you can get them for cheap.  A great example of this would be the big S-class coupes.  But the downside is there is a reason you can get them for cheap: they're, to coin a phrase, "endless money pits."

On this site, you're likely to get the usual Scotty-Kilmer "get a lexus or acura" spiel.  Good advice, but the problem is that most older/cheaper versions of those brands don't exactly, ahem, make you look "rich".  They're just a little too sensible, a little too plain-Jane.  But here's my suggestion:

• Lexus GS (whatever year you can afford): they're long and low and look classy even in the out years
• Get a dark color
• Get the windows tinted as dark as your state will allow
• Get the paint corrected and wax the crap out of it
• Get the classiest aftermarket wheels and tires you can

Or, if you want a cheap conversation starter, go old school:

• VW Karmann Ghia (yes, you read that right)
• Do the minimum to get it running and driving (won't be expensive)
• Make sure the body is straight, straight, straight
• Silver paint
• Oxblood interior (vinyl or leather, whatever your budget will allow)

I know that's maybe a little too "out of the box", but it gets you the Porsche Speedster aesthetic for a fraction of Porsche Speedster money.

Good luck