Wow, 250K miles?
That sounds steep to me. I need more info. Has the engine and drivetrain been rebuilt or replaced?
The cooking side of this truck is probably workable, but if you want to make a living off of it, this thing better start every day, and end every day running.
I would personally have a hard time paying 10K for anything with 250K miles on it.
Don't let your desire to follow a dream cloud your judgement. Get a PPI (pre-purchase inspection) done on it. It will cost you at least a hundred bucks (most likely 200 bucks for a really thorough PPI), but you really need to know what you are facing down the road.