For conventional oil i would definatley go with Rotella or Mobil. The Cummins reccomended oil is fine too.
Personally, with 52,000 miles on the truck already, i would not invest the money in synthetics. One of the greatest gains of using Synthetic oils is the reduced engine wear which in turn will mean longer bearing life, less oil leaks, etc. With 52k miles (And depending on the owner, they could have been HARD miles) alot of that wear has already occured. So you may only be able to get the gains that you would if you switched to synthetics at say 15K.
I have seen Cummins and Detroit Diesels in tow trucks (that are abused) with over 400,000 miles on them running conventional oil and getting oil changes at 10k miles or more. If you can afford it, go Synthetic, otherwise conventional is fine with oil and filter changes every 7-10k miles depending on how hard you are running the truck.
Its almost a 2 for one difference in price being that 12 quarts of good synthetic oil cost as much as 24 quarts of Rotella. Personally, i cant justify the expense of synthetics so i can go a few thousand more miles between oil changes. Its pretty easy to change the oil in a Ram, you can almost crawl under the truck to get to the drain plug.
This just my opinion from working on diesels for the past 11 years. Good luck with your new truck.