OEM or go home. Im not trying to be an OEM snob but things like water pumps, alternators, CV axles ect.... are all garbage when it comes to reman's and cheep replacements.
Whatever you get, get one with a metal impeller and a RUBBER, not quark, gasket.
Thermostats are all about the same.
ID parts is probably the place to go now.
Parts for our MK3's are getting harder to find and that means that either you get cheep crap or only the good stuff now. Nothing in between much anymore.