As was mentioned previously, something in the 40-50 hp range should serve you nicely. A smaller tractor will pull a 4-row planter, but the weight of the planter if it is a 3-point would make a smaller tractor very, very light on the front end. Same goes for shredders.

I would suggest looking at something along the lines of JD, Massey Ferguson, Case IH, Ford, Kubota, or the likes that is still in business with a diesel engine. Oliver and Minneapolis Moline made some decent tractors, but parts can be awfully hard to find depending on what breaks.