H1's are somewhat of a niche market vehicle.
The trucks are currently holding most of their value well. The bottom hasn't hit totally yet, and some owners have sold them for small profits recently as well.
The biggest depreciation for H1's occurs in the first 1-3 years of ownership. A great case in point - a 2004 H1 that normally went for 120-130k sold for 77k last month. This was a dealer demo that had a mere 1200 miles on it. (The new owner gave his H2 to his 16 year old daughter for her first truck).
Once past there, things seem to be stabilized. 96-98 trucks seem to go anywhere from 30-48k, 99's to 2001's go higher, and 2003/2004's are higher than that.
The best database on used prices so far seems to be Ben William's EBay database.
Right now, for those that sold, the average price (for 1996 to current trucks) is 41k. This is for 251 trucks that have actually sold on EBay between 2000 and 7/19/2005.
The other major issue to look at is what you're going to use it for. If you're more interested in off roading, then look for a 96 to 98 model year, or any model year with lockers (ARB, Auburn, Detroit, or Eaton E-Locker). If you're looking for more of an all around vehicle with less off roading needs, any truck would work.
The two best links to look at are:
http://www.flashoffroad.com/features...ayAuctions.htm
http://www.flashoffroad.com/features...uyingUsed.html
Getting a Duramax is very hard to justify. However, we will see pricing drop on them as well, possibly with the same amount of depreciation as what we have seen with the 2004's.
As for conversions, they are a bit expensive. Depending on how you want to do your conversion, a Duramax conversion (with donor truck) could cost you as much as getting a new Alpha that's used.
Just my .02....
-Steve