7th Sign, I think Argonaut has a point. Instead of trying to get a car which just looks cool but costs a lot to run, maintain and insure, maybe try finding a more reliable normal sedan. I'm not saying this in an offensive manner but more from a practicality standpoint- you have a budget and you're probably a P Plater. A lot of the "good cars" in the under $12k budget probably have serious issues and will have huge repair bills.
But since you are asking for feedback on the cars, and in particular, the 300zx; my cousin uses his Fairlady primarily for racing but as others have mentioned, is plagued by reliability issues. Half the time I'm there it's sitting in the workshop being repaired. Whilst it was his first car, everything has been replaced at least once inside. However, if done up, it's a performance beast- but then considering we live in Australia and you seem to have a budget, I don't think that's something you'll be considering.
I'm not sure if you'd want an old C180 Merc- they don't look too nice in my opinion and definitely won't suit the image you're trying to give off I'd say. My mother bought one in 1999 and it was horrendously slow. A 318i might be a bit better in terms of insurance and in general driving (they do bill BMWs as drivers cars), but I also am inclined to think that an under $12k one won't look too flashy either. However, I still would pick the BMW out of the options you presented- I've got a soft spot for BMWs^^
... which leaves the Toyota Soarer. That's the Lexus SC400 correct? It's quite a good looker and if it's a Toyota, it shouldn't have as many reliability issues as the 300zx. I would probably pick this car over the others if I wanted to make the most rational decision.
In response to other posts about the Lancer and RX-7. I can't see the website (my net is stuffing up) but I like RX-7s; not just the new(er) one but also the old FC is quite attractive. Lancers, they are often done up too much to look like Evos. Ironically, a real Evo looks very plain and simple- but it too also had reliability issues. My cousin's Evo7 had problems with the stock Transmission/Gearbox and had to get it replaced. So if that's supposed to be the performance model, I'd hate to think what a normal Lancer would be like.