"I'm a grown up and I like to have fun?"
Mazdaspeed 6
May not quite fall under 400/monthly when insurance is figured in. However, this was the car I wanted to buy. Problem was that none of the examples I drove had been taken care of. Power delivery is pretty standard fare for a small displacement turbo. I wouldn't say it's all or nothing, but I remember the most fun being between 3 - 4.5k and major boredom everywhere else.
As for easily falling under the benchmark:
Mazda 6 Wagon V6 5spd. Or sedan for that matter.
I was shocked this existed in America. Ultimately, it doesn't quite pack the punch that most other midsize V6's do. But it looks better, and drives better. Maybe it's a bit to mom mom and not enough zoom zoom but it was on my radar when I was looking to get four doors.
Lexus IS300, Subaru Legacy GT, Infiniti G, E46
Previously mentioned standard fare in this kind of discussion. Though, mostly endorsed.
Volvo S60 T5 maybe the R?
You probably won't get noticed. Unless you're circle includes people who like academics. But, it's a very underrated player in many of these talks. It's very nice inside. It looks good. It's not a corner carver or drift champ, but it gets the job done and it's deceivingly quick. It's not really far off the R, which I may as well suggest, but decent examples with good prices are hard to come buy. They are out there, but not on every corner.
Saab 9-3 Aero
I admit I know little of this car. 2.8L Turbo V6. Front wheel drive. Can be had with a 6spd. Oh, and I personally think it looks better than anything else I've mentioned.
CTS
Fits the parameters. Not much else to say.
Less grown up options?
GLI
I'm not interested in this car. But hey, it's an idea.
WRX
Was not on my radar and as mentioned, it's probably not worth the headaches given by previous owner.
GTO Alternative?
Mustang Mach 1
??