Addressing things in order:
The condition code is assigned by the military unit that turns in the item, no one else. By regulation, it cannot be changed by anyone else regardless of the "real world" condition of the item. Units are specifically cautioned to not under-rate property but in more than 30 years of experience I've never heard of a unit catching hell for what it claimed as a condition code.
HX translates, roughly speaking, to "junk": however, the real condition could be, and sometimes is, that the property is in the factory packaging.
HX is a convenient code because some of the codes that indicate a more readily usable condition require the surrendering unit to turn the item in in "ready to issue" condition. That would include all of the scheduled, anticipated or required maintenance and turning it in with serviceable tires, canvas and tools etc.
Given a limited budget for repair parts and labor man hours, HX may make the most sense for a losing unit that is not sending the property to a sister unit. I'd be lying if I told you I haven't used myself.
Regarding purchase and export: GL's website does not mention citizenship as a requirement for eligibility to purchase property.
Additionally, the EUC form has spaces to include appropriate export info.
I don't know the law on vehicle export but it seems that the correct place to start with an straight answer to all the the above questions would be to request a written determination from the Defense Logistics Agency (DLA), specifically, the Trade Security Control (TSC) folks. The DAL TSC would likely be the final and only arbiters of the whole question. Try starting at
www.dla.mil
Lastly and, perhaps most pertinent, will Canada allow importation by or to a private citizen?
Hope some of this helps.