I noticed a very tiny

I noticed a very tiny fluid lead under my cavalier. It slowly got worse, then evaporate. (at first it was very oily feeling)
Everything seemed fine as long as I replaced the fluid accordingly. The temp. was always fine.
Limited to repair possibilities. I irresponsibly kept driving it. I filled it too full twice, because it started leaking so fast. That was bad, and the car started to get hot for the first time. I stopped and slowly released the R. cap to release pressure and let fluid run out a little. As long as I have the air on and I'm accelerating. The temp. stayed fine.(by now you know I'm an idiot.) The next time I drove it, nothing I did stopped it from getting WAY hot.
That IS THE LAST TIME I'VE DRIVIN IT.
I've priced both a radiator and a water pump. I think I like the water pump scenario the best. If it is the radiator. Getting one from a junk yard would be counter productive if it's the same year car wouldn't it? Or would that be O.K. if it was from one that was only actually driven for a small amount of miles for one reason or another. Then rendered lame for parts?
I did have a friend look at it. And after the engine and what not dried from me filling resivour. He notice a little rust stain where the water pump it, then the leak. I hope my question is explained well.
thank you