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We have 5 acres and 3 horses. You never want to have more than 1 horse per acre, or you will over graze your land. Rotational grazing (divide your property) helps a lot as well, horses can be picky eaters and if they have enough room will only eat what they like and leave the rest high. If you rotate smaller pastures they dont have as much choice and will eat more of the grass and not just the stuff they like best. If you are feeding hay, one good 60lb bale will last an 800lb horse 3 days. (Unless you are quite large yourself, a 14-15 hand cobby horse is really all you need)
I dont know about the UK, but the majority of our yearly vacines can be given yourself. That takes out the vet visit, if you do need a vet for vacines you can talk to others around you and split the cost of a farm call and do everyones horses at once.
Farrier is about $35 here, and I learned how to keep their feet rasped and shaped myself, which spans out the time between professional trims.
Grain depends on the horse. I have 1 that really needs it, 1 that gets a small scoop and 1 who stays fat on air.
Geldings vs mares. Unless you plan on breeding (I dont recommend that for the most part), geldings are a better option. Mares, like women, get hormonal and can turn witchy as hell around their heat cycles. Unless you have a good mare and can afford a good stallion, I wouldnt breed. The economy is such that there is no guarantee you will be able to sell the baby for any more than the stud fee.
Tack can all be bought second hand and still be good. I love tack auctions. They're like crack for me.
One horse can be fine, but they are herd animals and do best with a friend. A sheep or goat can suffice
That being said, one or two horses is not hard to maintain. Just don't over extend yourself and get too many. I probably spend about 200-300 a month on average on my boys, there are always variables but thats what I spend