Have a look at that '98 again.
Despite some info given, the 12v is actually the one prefered when you are looking at '98's since the early part of the production year used the mechanical 12v 6BT and the later part of the production year with a mid season switch to the new 24v electronic ISB. In most cases, a '98 12v gets slightly better mileage than a '98 24v and you can get 300HP out of the mechanical pump for nearly free.
The nice thing about a 12v 1998 is Dodge updated the truck itself in 98 that included a new sleek interior, new aluminum radiator, quad cab with 4 doors, and more stuff that I probably don't know.

Yet, you still get the mechanical motor that is really reliable and many folks still like better.
If you go with a 24v truck, go with the latest one you can afford. There were a bunch of bad engine blocks , code "53" that were cast incorrectly and left with too thin of a wall and eventually it cracks and leads to a coolant leak. Usually the only real fix is a new block.

These bad blocks are found randomly in 98-early 01 trucks.
The 2001.5 is likely the best 2nd gen Dodge Cummins out of the 94-02 run. It had the 235HP programming (98-00 are 215HP when equipped with an auto) and rear disk brakes where 2001.0 were still drum. '02's are a good choice since they have electronic cruise control where everything pre '02 uses your vaccuum diaphram. The thing I don't like about '02's is that Dodge randomly started removing items from the cab and going "cheap. My '01.5 was quieter than my dad's 02, I had a hood blanket, dad's didn't. They changed the vanity, I have a nice flip down mirror in the visor, dad doesn't.
'01 was also the first year for the high output Cummins that had the NV5600 6 spd behind it. My dad has one of these ('02 3500) and the NV5600 is MUCH better than the NV4500. It's stronger, no problems, and has a tight shift pattern like a car compared the 4500 that is like rowing an 18 wheeler. If you want a manual shift truck, get an H.O. if you can afford it and find the right truck.
Here's the scoop on the autos. They have a bad rep because they were just strong enough to stand up to a STOCK Cummins pulling RATED loads. So.... when people started hopping up the 12v's AND pulling around goosenecks with backhoes on them and totally shredded the stock trans, born was a bad rep by dummies that couldn't understand how the mechanics of machinery work.
Stock engine, stock trans, change the oil in it every 20K and don't drag around D10N's with your Dodge and your auto should last 100-200K.
If you do buy an auto, you just need to be in the mindset that *when* you mod the motor, you'll eventually need a trans with some good stuff inside. Figure $5K ( no, that ISN'T a lot of money, the Allison and Torqueshift cost even more, these aren't your Th400's and C6's anymore.

) and you'll have a trans that should handle 500HP, accelerates the truck faster, and can run circles around the Dodge guys wasting time rowing gears.
Mileage has SO many variables to these trucks. Some are basically worn out at 100K, and some still look new at 100K. I've seen dairy and farm owned trucks that are basically wrecking yard bound at 80K, yet my Ram at 161K looks about as good with a fresh detail as it did when it left the dealership, and runs better too.
You'll have to decide if the mileage matters or not by how it's been taken care of when you go and look at a potential purchase.