Don't fool around with the seals, they very seldom work for long and they can gum up things like thermostats. Pull the radiator and take it down to be resoldered. The price they quoted you is probably for dissassembly, boiling it out, rodding it out (Very good thing to do!) and then resoldering the entire thing with a pressure test. Do it right, your engine is worth it.
Check back with the shop to see if all this is included in the quote. Stick with the brass radiators, they always fail gradually not disasterously like the plastic tank ones.