You can't really fix the plastic tanks on the stock radiator. Some aftermarket radiators (stock replacement, not Koyo, etc) have metal tanks, which are well worth it. If you're really desperate, you might try some epoxy. Remember though that the radiator fluid is around 14 psi, and roughly 200 F. That's going to have to be some serious epoxy.

I had a similar rattle, which turned out to be the heat shield on the exhaust B-pipe. One of the heat shield mounting tabs broke off, and that corner was rattling away.

As for other parts to replace, my clean, low mileage 93 coupe needed 4 tires, all brake pads, TC bushings, tie rods, a decent set of seats, a working antenna, AC compressor, and a new driveshaft. It still needs 2 new wheels, rear rotors, AC recharge, and a pair of working speakers. Things that I over did it on, but still needed were a new shift boot, Nismo knob, and S14 seatbelts. I'd say that mine is fairly representative of most 240sx's out there.