Well, the HDX didn't hit the market until 2004. Prior to '04, there was simply the HD. An HDX will have a Cummins (there are 2 I believe) or Benz (which I would skip). The tranny will be Allison-supplied; one of the modern auto 5-Spd/OD (with unlockable 6th gear). Any of the Cummins would be fine with the Allison tranny. I'd look for the ISC over the ISB, however.
The real problem is that HDXs are bound to still be in service. If you find one for sale, it's likely to have a serious problem(s) or be really expensive. All the desirable features are options; i.e. White roof, underbelly storage, air ride, air driver's seat, etc. They are produced in many different configs. Of which, pickings will be slim for a couple more years, I would think.