Do you happen to know if it is an early year or late year model 1999? That year had two slightly different engines, nothing to take away from the reliability of the 7.3, just slightly different.
But besides that, the 99-03 7.3's are known for their reliability. I'm glad i got mine....don't regret a thing about it. Try going to the 03-07 6.0 section and hear the same from those owners.
These years aren't the fastest, aren't the strongest, and aren't the best on mpg's, but they'll start just about every time as long as you keep up with the maintenance. Mine gets a little fussy when it gets cold out, but a few cycle of the glow plugs gets her going every time.
Just be smart before you buy, check over everything. Look for the obvious things. Leaks would be the first thing i do. Just squat down and look under, any fluid and I'd probably pass. Mine leaks slightly but that's my fault....forgot to use a washer with my last oil change
. Also if you can, look under the air box (it's held on with two clips) and see if there is any dust on the intake side of the air box. One common problem with these trucks is the air box not holding tight and allowing dust to get through to the engine....unfiltered. No bueno
All in all, if you gave me 10 grand and told me to buy a truck today, the only truck i would consider is a 99-03 superduty....that's how confident i am in this thing