ALL Nissan 350Z's made prior to 2004 (do some research on the exact date) have a suspension problem relating to tire camber that causes tires to get chewed. The inside of the front tires is done within 10K miles. And with expensive tires, that is a big problem. Nissan dealers were offering to cover the cost of new tires, but it doesn't help that after about 1,000 miles it sounds like you are driving on a rumblestrip. Nissan claimed to have fixed it, but I would do serious research on Edmunds.com and some of the other car sites. You definitely do NOT want an older one, and you may very well not want a new one.
FYI, the Infiniti G35 coupe, with which is shares a chassis, does not have the same problem as the 350Z as it uses a different suspension.
The RX-8 supposedly has a problem with burning oil, just like all rotary's. The new sideporting was supposed to improve the wiper seal, but it still needs about a quart of oil every 500 miles or so, from what I've read.
The Honda S2000 is the best sports-car in that group. No flaws. It has the best stock gearbox of any production sportscar. ANY.
The WRX STi is fast, but Subaru's don't have the same reputation for longevity as Hondas.
The new Lexus IS has 306HP. And Toyota is legendary for quality. If you want a hardtop, get that. If you want a little convertible, get the S2000.
The Lotus is a head-turner. It uses the 4-cyl from the Toyota Celica, so it should have good reliability there. That might be something to look at as well if you want to be a little different.
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