Don't buy that pricey thing. That price for one with 150k, yes. With 240k, no.
The problem wasn't with the Triton, but the 5.4 in general (the early 5.4's weren't Tritons) and covers 1997-2008.The primary problem is the plugs would either break or someone would strip the threads out of the block (aluminum blocks). That's why you pay to have plugs done on 5.4's...if your mechanic strips the threads out it's his problem.
Now for the price, you can find 3/4 ton diesels for that. Granted slightly older. I've seen a lot of first generation Duramaxes (LB7) going for around $8000 with 250k miles on them. We use to run about 23 MPG in them (but there was the injector issue...). Another option is the 6BT (12-valve 5.9 Cummins). One of the work trucks I used to drive was a 1998 Dodge Ram 3500...it was purchased with 850k and it wasn't retired until the odometer read (1,)345k. It ran about 19 MPG. The first gen ISB (24 valve 5.9) ran about 21...
If you are looking at higher mileage trucks...look at the diesel market on late 90's and early to mid oughies trucks.