I agree that an auction truck is a chancy investment if you are not one of those folks that already know how to do a good quick inspection / evaluation on the value of the unit, and you can bet there are already at least a couple of folks at that auction that are pros at that, and are not going to pay as much for a unit that they can not fix and resell at a profit. You will not get the value they would from the unit. They know what it costs to fix and what is not worth fixing, and they don't pay as much for highly skilled labor, since they already have hourly service techs on their payroll. The ones not worth fixing will be the one you find within your price range that is -left- for the auction.
I had thought the same as John about the weight, but I went to the "spec checker" here at forklift action, and checked a couple of current 5000lb cap. units and they were below the trailer's capacity, but I would still expect trailer load weight balance to be a problem, and would not want to be right in front of your truck hauling that forklift down the road, stopping quick may be a "learning experience".
The mast inspection for Canada most likely is a bit more strict than here in the USA, and for the most part, while the front end (lifting section and attachments )inspection is called for by OSHA, unless someone gets hurt or something breaks, there will not be any sort of checking done to insure you are getting a proper inspection, at least by USA Govco.