The Troy-Bilt Z and the Cub Z are basically the same unit with different paint and decals, both made by MTD.
The JD rider is not a real JD, probably made by AYP or MTD. A real JD will say "Made in Moline" and hold its value.
Probably the weakest point in all of the three you mentioned is the hydro trannies. They don't have a filter or drain plug, and you can't change the oil.
If your friend is a careful driver and takes good care of his truck, don't gun it out of the blocks, is a stickler for doing the maintenance on all his stuff, that tells you something. If he ain't, that tells you something too.
If you just have to have a rider you can't afford commercial. A Lazer for 3K is pretty hard to find, I never seen one.
You have to weigh everything, make your business decision, and hope for the best.
I went with the wb, new, belt drive, eXmark Metro 32. Very reliable. Got a heck of a deal on it -- I think because everyone wants a rider and it had been on the floor quite awhile. It was right for me, but not for everybody.
308 Don't Hesitate When They Riot In My Town