Chris
I think you need to think outside the square here for a moment. You have a 50D?, on sell that to your dad and then that leaves you needing a 7D or 5D mk 2
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In all seriousness Canon's 18-200IS could be the answer to a good walkabout lens for your dad. I have one and love it, it seldom comes off my 50D and it is very lightweight to boot.
Wayne