Buick - Century :: 1999 - Overheated - Low Coolant Sign

-We have a 99 Buick with 38,000 miles. At 30,000 miles the car overheated and we had to find a mechanic in unfamiliar area. He told us car was unsafe to drive unless we had new radiator system installed (water pump, radiator flush, pressure test, radiator system flush, radiator kit (premium), anti-freeze - 2 gallons, lower and upper radiator hoses) at a cost of almost $1200.

FYI the car is in Florida full time - under cover.

Recently the "low coolant" signal has come on and after checking with a local mechanic, he said the manifold gasket was leaking and it could be fixed for about $600. We are reluctant to spend that money unless we can feel somewhat certain that the main problem is solved. We understand that at some point there was a class action suit against GM because of problems with Dex-Cool which is the required coolant for GM cars. In the meantime, we have been adding plain water to the reserve tank but are very concerned that this may cause additional damage. There is never any leakage under the car.