Need help picking out a truck for under $20k

I really need to be under $15k. Ive been watching ebay but I dont know squat. What I need is a day cab (but not limited too), could be a single rear axle and dosent have to be comfortable or anything fancy. Im a do it myself guy so I will be learning how to work on it. I work on my dodge diesel and build racecars. So while Im currently not a semi truck mechanic, Im no tard.

Im using the truck to pickup semi trailers (always empty) and delivering them. Probably never more than 100 miles when doing this and probably only a few times a month. Also just want to be able to move roughly 20 thousand pound items on a flat bed from time to time. Im looking for a very solid motor, easy, and lowest cost to work on (maybe older non electric controled). Whatever brand or year that motor comes in is what Im interested in. This is what I need the most guidance in. Picking a motor, truck and year

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I like the ABF site and prices look good. I need some ideas on what motor is best for me. Need something that is plentiful on parts and has good longevity. Cummins is a plus for me but required. What year and truck has a solid motor AND TRANSMISSION. Thanks

 

There are a good bit of places to shop but Im not sure what to shop for. Cummins or something with a maniacal pump? I dont know what motor and transmission to shop for. Any bullet proof truck, more, trans combos made?