Being ill and getting over it wouldn't necessarily rule one out for me.
Things that do rule them out include :
Mucky bum even when wormed
Repeatedly gets bad feet (once, recovers alone, no problem. Twice needs attention is out.)
Barges at me or past me, runs off away from the other sheep when being gathered.
Any issues with breathing
Narrow at the front - between the front legs
The biggest, chunkier ewes don't always make the best breeders. Often the best breeders are a little on the scrawny-looking side. But when starting out, I'd pick strong, calm ones.
I'd keep three, though, not two. If you have two and one dies... Whereas if you have three and one dies, the others still have company.