It all depends on the breed? do you know what they are? Personally I'd do for the two year olds if they have lambed before but I would ask if they had any problems? E.g. Prolapses, rejected lambs. Ask how many lambs they reared each time and what ever you do I would give them the once over. For both ewe lambs and mature ewe, look at their teeth. Are there any missing? do there jaws match up (over or under shot jaws) Check there teats, make sure they are in the right place and only two. Check feet and legs, feel there forelegs for fatty feelings, they should feel smooth and check there feet are hard, correctly formed, not deformed in any way.
you can breed a ewe lamb if its up to weight but there are some people who do not agree as they are still small and immature physically and mentally, so if its your first time having sheep as it sounds wouldn't be a good idea.
When do you plan to put them to the ram? I would dag there back ends out before tupping and get a FEC and worm if needed.