Go nismo, the convertibles look kind of ugly with the roof up. The oil cooler is necessary if you race, street driving you should be fine.
As far as your other 2 questions, idunno. Idrive auto and the Z is my first car
pssst: ditch the 'stang asap
And welcome to the forums
edit: if you're looking at performance, the 370z beats the g37. But the infinity is more luxury. 370z won that battle for me in a split second.