Is buying a 2004 Ford Expedition with 250,000 miles on it a bad idea?

There's a really nice Eddie Bauer Expedition for sale in the area, it has everything when it comes to features, along with four-wheel drive and the whole works. The man bought a new expedition and is selling his, it's really clean. I've driven it before and it runs and shifts great, my concern is that it won't be shifting great for much longer (Transmission). It has the original transmission, but it's been taken care of for a 2004.
1. Lets say the transmission were to go out, does anyone know exactly what that would cost to replace from experience in dealing with this before.
2. Are there any other problems that might happen with 250,000 miles and only basic repairs over those years (Like I said, from experience, no guesses).
Thanks
(The picture was taken in the mans garage where its always been stored)

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