The only problem with getting a hoist off of an old grain/dump truck is having to get the truck somewhere that you can work on, removing the old stuff, and hauling it around to put it on the new one. I'm sure that it is the cheaper route, but probably a little more time and work intensive. I would bet that the hoist would be heavier off of an old grain/dump truck than one from Northern or Surplus Center.
However, there is something to be said for the new units that might require less time/work and fabrication to make it fit. I ended up with a no name hoist off of eBay (bought from a guy who said he sold damaged/unclaimed freight). It ended up costing $1100 delivered and came with an electric pump. It still took me two months of weekends to install it and get it going.
Best mod I ever made to any vehicle.
If you do get a hoist off of a dump/grain truck, you could probably get it for "free" or at least really cheap by buying the truck whole, removing the hoist and other parts you need, and scrapping the truck.
Good luck and take care.