None GT
GT - Leather, Cruise, Bose, S Mode
GT Edition - Re-tuned dampeners and reverse camera
40th Anniversary - Coloured interior parts and dark Rays
Nismo - GT Version with an UpRev and a fancy frock
I only ever had a lowered 50 so can't really comment on ride quality difference. The car is more refined as a whole though.
One point to note is you will want an exhaust of some sort as it sounds more like a 3 cylinder than a V6 as standard.
You missed one important thing - handling on the Nismo is substantially improved (speaking from experience - much more poised around the corners etc.)
OP - you can't really go wrong with anything from a GT onwards in my view. I wouldn't suggest the non GT (very very few of those anyway) and you might want to go for 2010+ to get the better sat nav, but apart from that just find a well cared for one you like the look of and that's within your budget.
Ha I was only messing mate, I imagine there are quite a few improvements