Well, for my opinion....here it is.
The 240/260/280Z'z all had great classic styling, but were made in an era when Japan had poor quality steel. Rust is the death to many good Z's. But nowdays with welders and better materials...anything can be done to them. New frame rails are availble, along with floor pans, and if your good with metal working or composites...then they can be revamped into strong, durable, and hot cars. Some cars have faired well thru the years, and if you do find one where the framerails or floor pans have not rotted...then snag that car up, take everything off of it, fab up a rotisary, and blast the living f%^k out of it...get it back down to the raw steel, and repair what rust is there, and weld up some newer frame stiffining items you can think of...and paint, and slap the project back together. IT will add peace of mind, and also increase the life and value of the car.
As far as sitting in the Z's compaired to ZX's...When I had the stock seats in my Z, I had all the room from hell to sit, when I put in the Corbuaes, I lost a lot of room, but I can still fit somewhat decently in them...when I had a 280zx, i didn't have too much room, and it felt a little cramped. I'm 6'2"..so I a little taller than most people. Datsun specifically designed the car to fit an American that was 6' tall...it was one of their goals when they made the car. If you can, just get a re-foam and re-cover kit for the stock seats, freshen them up, and you'll like the outcome. Also, I've heard that CRX seats in a Z rock...I might attempt that if I get tired of my race seats.
As for wireing between the two, the Z will have less wires than the ZX's....and they are simple if you can get the correct wiring diagrams...I had to convert the 260Z wires with a hacked up 280Z wires...and that was the problem I had...but a little time, and they worked out good. The dashes in the older Z's come out decently, and everything is accessable when they are removed...the ZX's...are the nightmares...
For the swaps...anything can go into a Z, swapping in something into the ZX is a pain...less room.
For the power to frame ratio...if an old Z is fabbed right...it can take the power decently. They still are a very strong frame stock, but the main weak area's are the spots that are prone to rust. Fix the rust, and the car will be strong.
And aftermarket...Everything is out there for the Z cars...if it's not out there, then there is no need for it. ZX's have very little for aftermarket products, not a popular car to realy hook up like the Z car was. And by now you've had to have seen that parts and stuff for the Z cars are reasonably priced and are sometimes near dirt cheap, or given away....
In short, the Z car comes up as a winner. The limiting things are how deep your pockets are, how good are your mechanical abilities, and how wild you want it.
CRX seat in a Datsun Z