370z is an evolution of the 350z rather than a different car. JMHO 350z is old school, mechanical, raw,...370z is a bit more refined, bit more power, track little wider wheelbase little shorter, so probably a bit sharper, although 99% of road driving you'd struggle to tell a difference.
I've seen some stunning builds of both. I'd be happy with either. I'd say focus on a solid, well maintained, rust free, well loved example.
I buy a lot from Torqen never had any problems, decent guys; Tarmac Sportz also has a good reputation. I also buy parts online from the US, Japan etc. There is a huge zed sceen in the US.
I run an Infinity exhaust, fits well, great tone, not obnoxious.