Overheating at 160k or so is most likely due to head gasket failure, not a faulty thermostat. The OEM head gaskets of that period are prone to failure more often than not. The new replacement HGs are great and won't have similar issues. If you replace the thermostat make sure you only
use an OEM new Subaru thermostat. They are only $23. Head gasket failure is very common in the 2.5s right at the mileage that the GT is at. The cross drilled rotors should not factor into your decision to buy the car, in fact cross and drilled rotors are not any better than regular rotors, and if cheap are worse.
Fixing Subarus is pretty easy and straightforward once you are familiar with them. This trade sounds fishy and unless you are willing to tackle the potential head gasket issue I would walk away. If he has been overheating the subaru the heads could be warped as well and coolant and oil mixing destroying the bearings.
Has the timing belt done recently? If not, that should factor into your decision. If you do it yourself it won't be too expensive, but taking it to a shop will cost you anywhere from $650-1000. You need to know this because if your timing belt breaks your valves will be trashed and you will need to do some major head work at the very least.
Subarus are plentiful. If you really want one, shop around and do some more research. Good luck and welcome to the site. We'd love you to get out of a Honda into a Subaru!