I thought the KC-10 would have gotten the MD-10 cockpit upgrade package during one of their previous cockpit modernization programs, but you're right, the KC-10 still requires a flight engineer. Spare parts availability might be manageable since Canada is only looking for 5-6 aircraft and could buy out US's remaining inventory of spare parts and cannibalize from the remaining 53 KC-10, but this will get expensive and as you say, beyond maintenance costs, operating costs are also high so the KC-10 really isn't a good idea.My suspicion would be that the KC-10 would be horrendously expensive to operate, 3 man Cockpit crew for starters, which means the RCAF would have introduce Flight Engineers for them, thirsty Engines, and Spare Parts would be getting harder and harder to get hold of. And you would have to ask why are the USAF really getting rid of them, 59 Aircraft is not really a major issue Orphan Fleet wise, the USAF operate smaller Orphan fleets then that. The RCAF would have to train personnel on maintaining an old Aircraft. The Civilian Contractors would be using a lot of ex USAF personnel, who would be quite happy to do a 9-5 job before that doesn’t include all the BS that comes with being in Uniform to operate and maintain the Aircraft.
Canada would be far better off buying some 2nd hand A-330s and converting them to MRTT, A Number of close Allies are operating them incl Australia, UK, France, Singapore and the ROK, and the A-330 is still in production.
Probably a good time to pick up some 2nd Hand A330s in good condition too, might cost more in the short term but will be cheaper in the long run.