Generally, an early Boxster with a blown engine is worth about $3-4K. So, your car will depreciate from $6500 to $3000 over the remaining life of the engine. If it is still running at 180K miles, it will probably have reached $3,500 and level out waiting for the day the engine finally grenades.
I don't think that a 986 Boxster will ever appreciate in our lifetime. A lot of old cars are just old cars.