Budget for a full engine rebuild, most of the time the rings will loose tension and if you just replace the head gasket, it will always burn oil. The bores will likely be worn excessively depending on service history/oil quality used.
The usual culprits of failure on these p80 AWD engines is either oil leaking from rear cam seal onto the heater hoses, which swell and burst, or a coolant leak from hoses/radiator/coolant tank that was ignored.
You wouldn't necessarily need an entirely new head, but it all depends on how hot the engine got. I have seen people keep driving them and end up with a hole in a piston/seized engine.
Depending how much you can do yourself there's also the usual $2000 worth of suspension/stage zero stuff that a p80 usually requires to be reliable.