As a former and current Datsun Roadster owner, please allow me to chime in.
http://www.311s.org" onclick="window.open(this.href);return false; has probably the most complete body of knowledge on the little cars. There are members in Australia, Denmark, the UK, Canada, back east, out west and in the middle. Quite a few of the folks at risensonracing are also on 311s.org, but not necessarily the other way around.
The engine out of the Sentra SE-R can be made to work in the 311, but it will be a MAJOR project. The Sentra is FWD and the Roadster is RWD, so it will take quite a bit to make that combo work. The popular "hot rod" engine is the SR20, but there are also a number of KA24s being used if you like torque, albeit with a lower red-line. There are a smattering of other engines: V8s, V6s, Miata engines, CA18s, etc etc.
For a fun car to just drive around in, don't pass over the 1600 just because it doesn't have as many ponies as the 2000. The later cars ('68 +) have more "safety" features, but with the higher windshield they are uglier to me. The one linked to in this thread is way overpriced in my opinion. For $15 large, the map pockets should have elastic, not a sagging piece of vinyl on the door. No telling what else is wrong.
You should be able to find a decent runner for $3-6K. "Back in the day" (late 60s, early 70s), my 1600 was Cali smog exempt because it had multiple carbs standard from the factory. I'm not sure how they're handled today.
Parts are above average in cost, but the guy who runs datsunparts.com is very knowledgable.
If you're looking at a 60s sports car, the Elan is faster. But then it should be.