I'd forgotten about Romney and Finn. And the need to look for older lines....
If you're buying sheep, ask the breeder about how they manage them. If they're raising their sheep, regardless of breed, in conditions with a lot of grain, human intervention at lambing, etc and your system is dramatically different that could be an issue since the sheep have been selected to work in their system and may not work in yours.
I've purposely selected for "survivability" in my flock: grass and
hay with little grain, parasite resistance (sheep that need to be wormed often don't stay in the flock), easy lambing/good mothering, etc. I bought a few ewes several years back that physically had the characteristics I wanted (fleeces and conformation), but I ended up culling them and their offspring from the flock because they didn't have the survivability factor--the flock they came from fed a lot more grain, wormed all the time (so they weren't selecting for parasite resistance), and bottle fed lambs regularly (not the case here--it's really rare for one of my ewes to refuse a lamb even if it's a twin).