Nice project boat! If it runs and you can ski behind it then $4K is an ok price (although I'd try to knock that down a bit if possible)
If the trailer is really rusty, I'd think twice about trailering it home. Try to rent a flatbed trailer somewhere or use Uship.com
Even if the engine doesn't last, you could spend up to $6K more in repairs/cosmetics and still only have $10K into it and it will last you another 10-20 years with proper care.
Clean up what you can, change all the fluids, tune up, impeller, belt, etc and run it.
You may want to get a marine survey done since you're so far away. Good ammo to negotiate a lower price.
I was njskier on here.