When I Goggled John Deere cart the typical hit looked like this

I'm assuming that isn't what you want.  When I searched for John Deere grain cart I got a lot more choices, most of which I have never seen around here.  In this area when you say old Deere grain cart everyone thinks one like this
 
If that is what you are looking at, look very closely.  Most are worn out. (in fact one looking as nice as this would be suspicious around here.  It looks WAY too nice) There are at least 2 down sides to them.
1) you cannot shut off the grain flow.  Once you start unloading you can't fold the auger until it is empty.  Also starting with a full auger may cause problems.  If you can start the PTO slowly and gently you'll be OK. Wind up the engine and slam it in gear and something may give.
2) they do not have much reach. The auger was designed for a low sided single axle truck. A semi with high sides might be a difficulty. At one time we had a 12010a and a J&M 525.  To load the local elevator's semi you had to unload the Deere into the J&M.  It could not reach over the side unless conditions were just right, and they usually were not in the field.
3) (I said at least) The small wheels do not handle mud well.  Especially the slick aircraft tires they usually have on them.