I'll try to help some. You are looking at 2003 to 2010 models. For the record, there has been a lot of improvement in tow vehicles from these models to the 2017s.
1) A new 3500 SRW could probably handle the 14K toy hauler fine. The older the vehicle the less likely the SRW will be to handle it safely. The payload and tow specs have all improved markedly since 2003. In the age range you specify, I would stay with the DRW, unless you know you can stay within specs on one of the newer ones.
2) I would not be as concerned about this. You say you will move very week or so. If you move 100 miles or so weekly then it's no big issue. If you are moving 20K a year or more then get the axle ratio rated to handle the job well. If you plan on towing in mountains a lot, get the properly rated axle ratio. Almost all towing not in mountains it's less of a big issue. Can't help much more than that since I don't know your detail plans.
3) Where do I start. For the record I had a 2008 Ram with 6.7 and a 2015 Ram with 6.7 CTD. The exhaust brake is really nice towing in the mountains of Colorado. The 2015 had as much towing power with 3.42 rear end as the 2008 with 3.73. Power really increased between these years. The 2008 to 2010 years are before Cummins started using the DEF. The engines are prone to more emission issues than the new generation with DEF. The 2003 thru early 2007 5.9 models are prior to emission controls. That's a good thing. The bad things are that they are older and have less power. I suggest you research the changes in the Cummins from 2003 thru now so you can make an informed decision on this. Lots of pros and cons that you should consider.
Hope this helps some. Good luck.
