If you are going to go witht he 7500, get one of the other tranny's ie a 10 spd etc. Otherwise, the Fuller 6 is better than the ZF 6 spd behind the 6.6Dmax.
You can get both the 5500 and 7500 in a sub 26K gvwr ie about 25,500-26000.
The lower gears in the 10 sp etc, will give you more gradibility ie how steep a grade you can pull in first gear. My Navistar with an older NA 7.3 has a spicer 7 spd, first gear is 10.08-1, vs the fuller 6 spd at 8.5-1 IIRC, and the ZF at 5.64. Granted teh ZF has a OD vs teh fuller or SPicer, you will have taller axle gears with the direct drive rigs, but still may have more gradibility.
My Navistar can pull 30K up a 30% grade at idle with 336 ft lbs, The 6.6 with an ALlison, can do 24% at 20K, 28% at peak tq of 620 ft lbs, Th eZF with 520 ft lbs and the 6.6, can do about 10% greater grade than the allison. My 6.5 with an NV 4500 has more gradibility than a DA setup. Tranny gearing in rigs like you are getting, is as much a factor for slow driving and getting into sideroads, driveways etc, as the axle gear, IMHO more important.
The tranny can also make a bigger difference than the engine as I pointed out too.
Really look over the rig you get, if it is making you money, and you bought the WRONG rig and drivetrain, it WILL cost you more money in the long run. I did that when I bought a R3500/454/th400, that thing was teh worst pulling rig I had, a 292 I6 could pull a steeper grade doing what you are doing, I do in my landscape biz.
Do what I did when I bought my Navistar, ask for a gradibility chart, which shows how steep a grade you can pull with ea rig, with the tranny's availible, and make sure it will do what you need it to do.
Marty
MartyB
05 DW3500 D/A crewcab
2000 C2500 reg cab 350/auto
mem# 2825