I don't like the rope halters at all for my Primitive sheep. Maybe some of the larger breeds will fit them, but on smaller sheep they ride up into one eye and hurt the animal. I use Llugwy farm too - she does all sorts of sizes in flat woven polyester, various colours, and also posh showing leather ones. The leads are removable and the service is efficient.
I echo not to leave these things on your sheep for longer than you need to lead them, and never when you're not there.
You don't say whereabouts you are, or the breeds you are buying - it would help to know in answer to your anti-flystrike treatment. I live in Scotland and use Crovect, applied every six weeks during the fly season, judged by the weather, but usually May to October. We do only the lambs in May, and don't apply to the ewes until after shearing, as any of these products on the fleece will limit how you can make use of them. Also ewes tend to be less affected by flies than lambs early in the season. You need to keep a good watch though and act promptly if your ewes are behaving strangely, which could indicate strike.