Taylorce1,
I struggled with the same exact question over the last several months as my 1999 Super Duty is just plain old with 417,000 miles on her.
A similar new truck at $58,000 plus vs a used truck with 200,000k miles on it at sub $10,000 was a hard decision as I really wanted to go with the new one.
I ended up grabbing a 2000 Lariat for cheap and I gotta admit, it was a great call.
I know all the quirks/fixes and have the confidence to drive this old 7.3 to wherever the hell I wanna go at the drop of a hat.
Even if you had to put $8-10k into it, you'd be upside down on it way shorter than if you bought a new one.
Hard to lose on a 7.3 man.