Obviously, try to find a car with a 5-speed trans. The gearing on all the stock Rabbit gasser 5-speeds work well with the diesel (FF, FO, FN, 7A), with the exception of a few trannies (2H, 4K, FK, & FM). The 2H and 4K were stock on the GTI and are the WORST match for a diesel. The FK and FM came on some Jettas and Rabbits and are equally horrid.
If you get the whole car with trans, it will save you from 1) buying a trans, and 2) having to swap all the components for the linkage (relay rods, shifter box, etc).
Frame rust should be top concern, but in Cali, I doubt it will be a real issue. Floor pans like to rust, as to windshield frames.
Early Rabbits (up to 1979) are lighter than the later cars, so it will improve MPG.
The gas tanks are essentially all the same. You might want to find a diesel filler tube for the larger orifice to accept the large diesel nozzles at gas stations.
You'll also have to re-route the fuel lines. The gassers finish on the driver's side of the car, the diesels finish on the pass side.
That's about it. The wiring harness would be helpful for the diesel swap. too.
Good luck! I acutally did this a few years ago, but I had a complete diesel donor which made it easier. But it's not a really hard job. You'll want to replace all the bushings, shocks, ball-joints, CV boots, etc while you're doing the swap, as all those parts are sure to be wasted.