It is always best to buy a TC with all the attributes you eventually want, and not add on later. Many reasons. The 2003 / 845 is in a pretty good window for Lance. I have a lighter yet, less tall, narrower, Lance Lite 165-s, Xcab that has been a good trooper for me, especially considering the roads we go on. One of the successes has been that we have nothing on the roof. No weight up high. Don't discount the value of that aspect, especially if you are going off-road.
My only other thought is that a Dodge Cummins 2500 must have the suspension and tires upgraded to carry the load of the even modest 845. My '01.5 came with the 'camper package' from the factory which included upper secondaries (aka: upper overload springs) which was, defacto, Dodge's first entry into the land of the SRW/one ton, and an upgraded Dana 80 rear axle to 35 splines and a higher weight rating. Even with that, I had to incrementally increase the defacto (non-weight-police) load carrying capacity.
Go to this thread about my Dodge suspension build-up:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27572829.cfm
Good luck on your quest, if indeed there still is a quest.
regards, as always, jefe