Much good stuff has already been said, but a few things I would add.
You are in North Yorkshire so you have to think about TB testing. The vets can insist on a crush (although some are flexible so long as they will be safe), and this could give you a considerable headache if you go for Highlands, because of the very wide horns.
You can halter train at any age up to and including in-calf heifers, provided you get ones which are well handled to begin with. If buying unhandled, then yes, get them younger. If you can source from a local farmer / breeder, so much the better as you can see them at home, but as you have experience of cattle you would be able to tell what sort of temperament they have in the sale ring.
Calves with horns can give you problems to solve - dehorning / disbudding - unless you are happy to have all your beasts horned. If not, it is *much* easier all round to use a polled bull where, if your cows have or had horns, the polled gene is dominant. Angus works, is usually a good choice for ease of calving, and nice quiet, decent meat calves that will do well on grass systems. Or stick with Galloways of course, if you go that way.
Galloways come in lots of colours, and the Riggit is regaining popularity. Personally I love the look of the white, with the black points.
It suits our system here to send our meat calves off at under 12 months and not over-winter them. They are just for our own consumption and their mums are "cow-calf" house cows (so they rear their own calves and we take the surplus) but it wouldn't make financial sense in a commercial setup to send them off so young. However, the meat from these milk-and-grass fed happy young animals is awesome. We would of course get more than twice as much meat if we kept them for two summers, but for us that doesn't compensate for the cost and extra workload of overwintering them, plus we don't have a huge amount of grazing, so would struggle to have enough grass for all our cows and calves and sheep and lambs if we were keeping calves on to 18-24 months. But the key to having decent calves at 9-11 months is milky mums and, of course, a decent bull. So if you did think you wanted to send them off younger, I would suggest looking at a milky breed such as Red Devon, Shetland, a milking strain of Galloway (they aren't all as milky as some), Whitebred Shorthorn, even Northern Dairy Shorthorn. (The latter is more akin to a Shetland than a regular Dairy Shorthorn.) And / or a Jersey cross of one of those breeds. Our butcher raved about the 11 month stirk he butchered for us off our Red Devon x Jersey heifer, using a good Angus bull, and our Jersey x NDS heifer is making an outstanding job of rearing her NDS calf; he"s a good chunky meaty shape for all he's 3/4 NDS 1/4 Jersey.
Sorry, I do get a bit carried away!