Remember that a good used trailer is easy to sell. You can always sell the 7k and buy a 9900lb trailer, at least that is an option if your truck is capable of the extra load and you really want a larger tractor. I agree with the biggest tractor being a 3616 or a 4010 for that trailer if you have a loader and maybe an implement on the back. A 5010 with loader would be right about max, but by the time you fill the rear tires and add an implement you would be too heavy for a 4800# capacity trailer.