Each system has problems, but you've probably got 'em ordered about right. If they haven't already, original cd add-ons will eventually need a gear change and original Duos/SuperCDs will need caps (as will laseractive pacs), leaving the Duo-R/RX as the least likely to need help. But it doesn't really matter, as gears, cap jobs, and laser swaps can be done on 'em all. If anything, the Duo-R/RX are probably most likely to end up being dead and unrepairable due to their lower quality pcbs.
As for how long a refurbished unit will last - there are some units still in use that've never been repaired, so I wouldn't be surprised if they lasted for another ten years (or more) before needing help again.