There is a big difference between the first series 70-73 240's and the last 79-83 280-zx's in terms of comfort and technology.
Decide what you are looking for, and you are safe getting any of them except for the 73 240's (unmodified) and the 74 260's, they had emissions related carb troubles.
I have a 72 240, because I like the old models. But I would not mind picking up an 83 280-zx turbo someday, they are comfortable and fairly quick.
I have a 73 240Z... and to call this a problem is an understatement. The later production 73 Z's and 74 260Z's used flat top carbs so they could meet federal regs. These carbs aren't known for producing good flow rates, so the engine is "detuned". Most everyone I know has chucked the flat top carbs and have replaced with either the earlier SU's or have gone to a triple setup. So there ya have it.