I have Shetlands in their teens producing lambs off very poor quality grazing and looking well on it. They should last longer than other breeds but you'd have to go through them and take out any losing their teeth. I'd be surprised at Shetland ewes losing their teeth at 5 tho and their udders general are very good too.
I would expect to pay 40-50 for ewes, 30-40 for ewe lambs and my friend bought a 2 shear show winning tup recently for £80. Not sure where you are located but if you can buy from Shetland direct you'd get tough sheep, or there are special Shetland sales at St Boswells in the coming weeks (probably not all pure Shetlands, will be sheep FROM Shetland, but bound to be plenty of Shetlands among them).
You can cross them with something like a North Country Cheviot and get better lambs but they will still pop them out, the lambs are tiny at birth, even the crosses.
Shetlands are tough and simple little sheep. The only draw back with the breed is they escape for the sake of it, it doesn't matter what's on the other side, it could be moon dust, they will still escape if they can, gaps under the bottom wire or lack of rylock, you won't keep them in. Oh and you need a good dog to get them in or make sure they are very petted and follow a bag. They are greedy and quite easy to tame down. Easier than Soays or Hebs I have found.
But, if your fences are not good, get some Hebs instead. They are a bit dearer to buy but their horns will stop them escaping and you'd get a slightly better carcass from them.