If you have a hardy native like a Hereford, in the Midlands, and your ground isn't too wet, and you clearly won't be overstocking, you might not need to bring them in at all. We outwinter some of our natives and native cross cattle and we're on wet moorland/upland in the far north of England.
One beef cow rearing one calf on good ground shouldn't need much extra feed - just some hay when they can't get at the grass and maybe when there's little nutritional value in it in the late winter. If your ground can stand the footfall, I'd just have a shelter available for them if the weather turns really evil, and for bringing her in to calve if the weather isn't kind at the time, or you are anticipating problems.
You might want to give the calf a little cake through the winter and for a month or two before you sell it, just to 'keep the belly off it' - unless the market around you wants grass-fed, in which case stay off the cake. (Ask your local farmers, the auction company, etc.) If you will be selling in the winter, you might want to bring the stirk in the winter you will be selling it, to stop it growing too much hair and keep it clean - you'll get through a lot of straw to keep it clean, mind.
You will also need to supply minerals - ask your vet or local farmers what is required in your area. And med for worm and fluke, ditto.
One tip is to make sure you get the pedigree of the bull used when you AI her - some processors will pay a premium for cattle with a proven pedigree native sire (usually Angus or Shorthorn but I expect there are schemes for Herefords too), and this will be reflected in the price paid for your stirk in the store ring - make sure the auctioneer knows you have the pedigree for the buyer, and says so.
Another thing to think about is how you will never have one bovine on its own; if you have one cow and want to keep her calves on to 20 months, how do you wean the first without it and her being alone? Maybe it would be best to think of buying two cows, then they and their calves can keep each other company. I guess you could leave them all together and let her wean the first calf naturally - but then you wouldn't be able to bring the older calf in before selling it, you'd have to sell it off the field, or bring the whole family in. Food for thought, anyway.
If you do house over winter, budget on half a small bale of hay (or one bale if they're rather lightweight) per day for the cow, similar for the last year's calf and a little for this year's.
It sounds exciting - good luck!