1998 was the change-over year between the 12-valve and 24-valve Cummins.
The 12-valve is desirable especially as an older truck because it has NO electronics to fail.
Early 1998 trucks have the 12-valve, "1998.5" trucks have the 24-valve.
1998 is also the first year that the Quad Cab had opening rear doors.
The "Unicorn", most desirable truck is an early 1998 with the 12-valve and the opening doors. Hard truck to find, produced only a very short time. That's what I wanted but couldn't find a nice low-mile example.
If you buy a 12-valve there's just one fix you need to do, it's called the Killer Dowel Pin.
There is an alignment dowel pin in the front gear case that works its way out and falls into the timing gears. It's a super cheap fix, and absolutely must be done.
If you buy a 24-valve, you'll want a tuner for it even if you don't want it to breathe fire. The only one to consider is the Quadzilla Adrenaline. It allows you to custom tune everything about how it runs, all others use pre-programmed tunes and can't make the truck run as well.
I know guys will say "leave it stock or you'll have trouble" but after all these years, a lot of things have been already sorted out and there are a few things that truly make big improvements even if you're not a "hot rodder" type. The truth is if you leave one of these trucks 100% stock it'll be LESS reliable.