For an 04 with that many miles, I wouldn't pay that amount.
Now, I'd have no qualms buying one, provided it checked out via a mechanic. If the seller won't let you have a mechanic put it on a lift and check it, don't buy it.
The trans rebuild will cost you between $4-5k, most likely. Headers have been addressed above, along with axle seals. The only other thing you want to look at is the radiator, as that can be an issue sometimes, developing cracks in the plastic tanks. That'll run you less than $200 for an oem-style replacement. I wouldn't spend the $300-500 for an all-aluminum radiator for a truck with that high mileage. And you'll need to do rotors more often on the early models, as mentioned above.
You're looking potentially at ball joints, bushings, hoses and belts being worn and needing replacement on one of that age, as well as possibly front wheel bearing assemblies. Nothing difficult to do, but it can add up quickly when you're paying $50-200 per issue, and you have a series of issues.
You should be able to find a decent truck with less than 150k miles on it for $10k or less, I suspect. It will take some shopping, but they are out there.