I have been feeding my goats Dumor feed since I got them. I have now stopped. Before buying any feed, always check the seal area, generally at the bottom where the ingredients/contents label is located, for the packaging date. TSC is notorious for over stocking feed, that is kept in NON climate controlled areas in their back room, and the feed can go bad after as little as 6 -8 weeks. I just opened one of the last 2 bags I bought from them to mix with my animal's new feed and found moldy clumps of pellets inside. This feed was bagged in early August. When I bought it, I had no choice at the time as I hadn't switched feeds yet, so could not adjust them to the new stuff. They have a 1/2 pallet of the same feed stacked 15' up on storage shelves in back that dates to early July. I refused to buy any of that as I knew/expected it would be below grade quality.
In proper climate controlled conditions, yes, feed can last longer and still be healthy and viable, but when the feed sits exposed to temps above 100° and high humidity, not so much. If you have a farmer's Co-op near by or other dedicated feed mill, contact them and tell them what you're looking for. The Dumor I was buying was $14.99 for 16% protein. I told TSC that was too expensive and I was going to buy elsewhere. I now buy and feed 17% protein @ $9.65/bag. I buy it 12 bags at a time as the feed mill/store is ~45 miles away. Even with the cost of gas I'm saving money and getting better feed. The bags I recently got on Oct 6th were packaged on Sept 26th (Think I recall). Very recently regardless, so I know they are fresh and high quality.
Though I have goats, I believe it's pretty much the same with sheep. The medicated feed is primarily for the babies as they are starting to eat pelleted feed, to help prevent coccidia outbreaks. Adults don't, or rarely need that and many who plan to eat the meat, don't want medications and antibiotics in the meat. There are withdrawal times after administering meds. Even though they say it's safe, I prefer non medicated feeds. I've never treated for coccidia and never had an outbreak or issue. This through 2 successful kiddings. Not a lifetime of experience, granted, but should I HAVE an outbreak, THEN I'll treat for it, not before. Just MHO. I DO however recommend the CD&T vaccination annually for adults and initial + booster for newborns.
When you get your sheep, ask what the present owners are feeding and try to start them off using that. Any time you change feeds you should do so gradually so their rumen has time to adjust to the change. You can feed them whatever you choose. I believe with sheep a generic 14% all stock feed would work or you can go with the sheep pellets. Just DO NOT feed them any type of goat feed as the amount of copper goats need is toxic to sheep and many goat feeds have added copper for that purpose. If you read the label, most goat feed clearly states do not feed to sheep.
They should have hay available to them basically every day, even when they are on pasture, it's sometimes good for them to have hay as greenery is mostly water. Mineral is another thing they should have free choice. Many sheep do just fine on nothing but pasture. Others require a little extra in the form of pelleted feed. In a dry lot situation, you really will need to provide that extra nutrition from feed in addition to just hay. The quality of the hay will have a bearing as well. You can adjust and add or subtract by tracking their body condition. A starving animal is pretty easy to recognize as is a fat one (mostly).
We have had sheep for a year now with 24 ewes and Ram and 12 new lambs. We go to a local feed store and have them grind our feed. We buy 500 lbs at a time and they bag it into 100 lb bags for us.