Well it looks like you want a rear drive car. Out of that batch I'd have to say get the 300ZX. I think that it would be the most modifiable, but on your budget, you'd spend the whole amount just buying the car and have nothing left over for modifications. I don't know much about the 240SX, but from what I know you need a turbo on it to make it perform (ie. Silvia in Japan). Another option is an engine transplant for that car (SR20DET from a Japanese car).
As far as the Eclipse goes they are good cars. I imagine they would be expensive though and I don't like how they handle. Basically I think they are tanks for the size of the cars. They are easy to modifiy and there is a huge aftermarket, but I'm not into them at all.
Truthfully if you want a great car to modify from the Nissan lineup go buy an early model Sentra SE-R (91 would be the year to get). Those are pretty cheap now and they have a great engine to work off of. The tranny is a little bit of a problem, but they have a stock limited slip. Problem is they are front drivers.
There are a lot of other great cars for project cars, but it depends on what you want to do with it. Are you going to Drag Race it, AutoX it, show it, or just enjoy it? That is my 2c's.
