If you believe you will eventually go for a fiver, skip 2500 series and go with a 3500. Newer the better. What is your budget? Fords last good motor until the 6.7 was the 7.3. But those are getting really long in the tooth and have really low ratings. Dodge Rams have offered the Cummins for years, with only one major upgrade. The 6.7 was a big leap in all aspects. Better power, cleaner burning, way more standard features. Add their wonderful 68RFE 6 speed auto and it has proven to be hard to beat. Think plan ahead. How big of a fiver do you think you will go for? Plan a truck to handle that much. It may be overkill today, but unless you want to buy another truck in two three years, get what you need tomorrow now. And dont discount a dually if your going for a fiver 35 or more feet.