Shetlands are a good point, unregistered especially, but you would struggle to get 70 or so of them from one source. If you have to cobble them together from different breeders you will need to make sure they are wormed/fluked/scab treated etc when they arrive, otherwise you would just have lots of problems in your first year. See what price they are on the Isles just now? If reasonable price you may get them shipped, and you could have a larger group from there.
If you get good sized Shetlands you could use a larger tup to get "heavy light" lambs without any additional feeding for late Nov/early Dec sales. (Thats what I do with mine - last year 1.80/kg). And they would amost certainly have twins in the majority, and only need minimal feeding and really only rarely require assistance when lambing, even outside (which is what they prefer). But you need good fencing!
PS.: I have none for sale, as I am having to breed all my shetland and X shearlings given the cost of buying in new larger breed/commercial ewes and I am in a similar situation in having to increase the flock or loose a field...