Is it a 6.0 or 7.3 as they were both offered in 03'.
The 03' 6.0 is FAMOUS for having several issues. The EGR and throttle plate to be exact.
The 7.3 is reliable and a 500,000+ mile engine that requires nothing but fuel and oil/fuel filter changes.
$20K is a little high, but if it is in great condition it can be worth it. I paid $19K for an 05' 6.0 CC DRW Long bed (friggin aircraft carrier) with 80,000 miles on it.
If the truck you bought still has under 100K your motor is still under warranty.