Well hip, where to start.....
The 240 could be considered the Cornetto car....the original and best....but more original than best.
All the successors benefitted from the experience gained and are therefore more refined.....BUT, Nissan also lost its way a bit in trying to build a car to suit US tastes, which meant it got bigger and slower and lost its roots as a sports car.
Z's went into the wilderness like American muscle cars (some of which had 600HP in 1965, but struggled to make half that by 1975) pretty much all the way through the mid 70's until the 90's. With the last generation 300zx's they built a good car again. I was an apprentice mechanic at Nissan during the late 80's and drove the California spec mid life 300ZX's, and they were truly forgettable. The VG Quad valve TT engines breathed life into the marque once again! They've got better since then.
So...no Z is practical, although the current generation are closer to it....buy a 370Z if you want the most refined and practical car. For a really practical car get something like a diesel Ford Territory.
If you are nostalgic and want to go back to where it all began, get a 240 and prepare to spend money endlessly to get it right and keep it right, noting that with all the money you spend, it will still feel like a car engineered in the late 60's and built in the early 70's. That's just how it is.
The other ones have their pro's and con's, but you get the idea of the trade-offs you need to consider. I have a 240, and was lucky enough to get it 25 years ago when you could get them in pretty good nick for reasonable prices. If I were starting now, and wanted the early shape, I would try to get a 280Z (same body shape as 240, but not offered as original equipment in Australia), or a 280zx 2 seater like what Simon has...they are a nice car, and a bit younger than a 240 or 260, so you get a better base car....hard to find though...Japanese import jobs.
Oh, the other thing is how much money you have to throw at it...don't take on a resto if you haven't got the money to finish it...otherwise, you burn up what money you have got and don't get any result for it...do the car you can afford.
Cheers
Jamo