durabilty of merc 650 lower unit?

[Yachtzee]

I am running a 1969 merc 650. I also have two more of these motors in reserve, but neither has a lower unit. I also see a lot of these on ebay and other places for sale but without a lower unit. I never see a lower unit for sale anywhere. I was considering a stainless prop for my motor, but if this is a lower unit that is inherently weak, that might not be wise?

Are there any opinions out there regarding the robustness of early merc 650 lower units?

Thanks for any opinions.