Pristine Colt Series 70's usually sell in the $1000 range, usually a bit more if it comes with the original box and papers. For a gun that has seen some use, $600 sounds pretty good. If you really want it, you should jump on it pretty quick. There will be plenty of Walther PPQ's in the future, and I assume that you don't have an impending deadline to pay it off.
"Si vis pacem, para bellum" - If you want peace, prepare for war.