That milky residue is relatively common on the M112/M113 series V-6's and V-8's. Had the car sat for a long period before you drove it? It is definitely more common on cars that don't get driven far enough on a regular basis to warm them up fully.

Head gaskets aren't much of an issue on those engines. You can always ask your mechanic to pressure test the cooling system to check for a head gasket leak.

I personally don't like cars that have been in accidents, but if you go in with full knowledge of what your are getting, that's cool too.