2006 Subaru B9 Tribeca 180k miles for
$3900, is that a good buy, is the mileage
If it's the base B9 then it's priced at clean trade-in at Nada.com This vehicle should be accident-free, good shiny paint, great tires and mechanically sound. there should be no cosmetic damage that needs to be repaired. If buying from a dealer the clean retail value is listed at $6,250 So based on the information you posted and if the car meets the above standards it appears that you have a fair deal.
Have a mechanic check it out. If the head gaskets are leaking it is an expensive repair.