What will your new breed be Janeislay? If it's a primitive then there's no point in trying to fatten them over the winter as they barely hold their weight, and any concentrates are wasted. We keep ours healthy over the winter with good hay, grazing if there is any, Tup&Lamb coarse mix when there's snow lying and a molassed licky bucket. We don't apply any fertiliser to our pastures, partly because it's expensive, partly because that's not how we farm, partly because the grass is usually under snow in Feb and March, and most of all because the lush grass produced by artificial fertilisers can cause scouring in the stock. That's appropriate for here, high up in Southern Scotland, but other regimes will suit different areas.
We send them off in August ie 16 months, by which time they are as big as they can get before there's the risk of 'taint' - ours are mostly entire males.
The extra cost of splitting the carcases is minimal, maybe as much as £2 each. I'll try to remember to check on Monday when we get our boys back from the butcher.
Particularly with a rare breed, if that's what you will be keeping, rather than send them through an ordinary market, you are far better to find a direct buyer for your meat such as a top-class restaurant, where the chef will appreciate rare breed hogget and will pay a good price (which he will recoup from his customers) Cutting out the middle man, especially if the middle man doesn't understand the special qualities of hogget, will give both you and your buyer a better deal.
We sell to friends and a small group of 'in the know' customers as we don't produce enough hogget to supply a restaurant, but we and our family do eat a lot.