We have Katahdin hair sheep! Best breed ever IMHO :) We have never had a wether, just rams and ewes and lots of lambs :) They are all easy to handle except one ram is a jerk.
If you have good pasture they shouldn't need any feed.
Some are parasite resistant, which is a plus but you can only tell that for sure if the breeder has actually done tests.
The questions you ask will depend on what you're looking for, but if they're good breeders they will probably volunteer a lot of info.