Hi Kat, I'll leave any breed advice to those of the forum with lots of experience.
That said, I may be able to assist with the correct term for "good lambing rate" - the word is "fecundity". I kid you not. Be careful with using it in front of children as you could find yourself answering lots of questions!
Not specific to ewes but an absolutely fab word none the less!
Good luck with your new venture.
We use 'prolificacy' which doesn't sound so rude but is more difficult to say
For sheep breeds which have a slower rate of maturing, the meat will be different - better in most people's opinion. It depends though on what you like - tender baby lamb or a fine grained more flavoursome meat.
Maybe you could buy a 16 month carcase from one of your Herdy-breeding neighbours and see if you like the taste.
Other breeds to consider are the primitives - the carcase is also small but matures more slowly. Our Hebs weigh in at about 15-22kgs deadweight at 16 months. Carlisle has it's own Rare Breeds sale so you have a wide choice on your doorstep. Primitives have a good pelvis width to weight ratio so are easy lambers to a commercial terminal sire - you get the carcase size you are looking for but the hardiness, thriftiness etc of the Primitive dams. Breeds such as Shetland, Hebridean, Manx have a fair prolificacy at about 160-170% ie about 2/3rds of your ewes will twin, the rest will have singles, with the occasional triplets if your ground is lush.
Some people say that the Primitives are difficult to manage, being a bit flighty, but we haven't found that (except Soay sometimes

) especially Shetlands which are very tame, can be readily trained to the halter, and have one of the best British fleeces around to add a tiny bit to your income (fleece for craft or tanned skins)