ive got a series 4 infini, had it for over a year, and i highly recommend getting a series 5.
a few things to be aware of from my experience
1. its old and will probably need a bit of stuff fixing before its all sweet, my car needed diff mount, gearbox mount, sway bar links and controll arm bushes fixed up. each time i fix up something it just gets better and better.
2. expect around 320km or so from a full tank of premium which is around 55l, some people get more but its mainly people doing highway driving.
if you can get a series 5 in good nick for 6k then definatley do it. just make sure you get one in good condition not a boonger