Hello mate welcome along, there are 3 different types of Z you can get
DE engine with 276bhp
revup which is 296bhp*(late 06 models)
HR 309bhp*(07 onwards)
All have pros and cons, I got my 2005DE due to lower mileage, cheaper tax and I believe a DE engine has more/cheaper mods.
If your looking for a newer model HR, if it has a dual intake system then it is a HR as some dealers list them as HR models when they are not.
My main concern when looking for a 350z was making sure services were completed upto date, check the oil pressure was normal and that the clutch was ok or had it been changed along with the usual tyres,brakes etc.