Are you against going to wider wheels because there are a lot of 19x9.5 and 19x10.5 out there (BTW Nismo wheels are that wide). The biggest question is what is your budget? Depending on how much you want to spend will determine your selection greatly. From what I've seen not a lot of companies out there make 19x9s, usually it's 19x8.5 or 19x9.5. 19x10s are easier to find. WRT you going with 20s, IMO I would not go with a skinnier rim. Wider rims = wider tires = more traction = more
I would read through this thread
[Official] 370z wheel fitment guide (pics/specs) only. It will give you an idea of what is available and what fits.
This is what I run:
Volk Racing Formula Silver G2s Type 2
19x9.5 +23 - Front
19x10.5 +22 - Rear with 10mm spacers
Yokohama NEOVA AD08R tires
255/35/19 - Front
295/30/19 - Rear