350Z: 287 hp, 278 ft-lbs torque. People are generally very impressed and happy with the power and the way they drive. Good handling cars stock. Torque throughout the power band.
There are aftermarket items for it, including exhaust and suspension. There will be a good market for more mods I think. The potential in them is hard to estimate - naturally it's harder to make more power from an NA than a turbo. There have already been a couple of turbo setups developed but I don't know if they're for sale.
I don't know of any defects yet.... try checking out
www.350zforum.com and
www.my350z.com/ for more extensive info on all your questions.
I think you will be happy with the car. I'd like one!