How much land depends on where you are.
Here, you could probably do nicely with 2 acres for one horse, but in AL you really needed 5, and taken good care of pasture too, as the heat is so bad there.
2 acres down there would mean you would need to grain and hay possibly, here you might be okay if pasture well maintained. If you don't keep it up? Nothing but weeds eventually will be in there. Plus, you need to maintain manure pickup too. Diving it into grazing plots will help.
Figure at least 2 acres per horse minimum, up to 5 acres per horse.
Minis would be able to get by with less, there are some mini owners on here, so maybe they can give specifics on needs.
As for electric fence? Too much worry about if power goes out for me to want it. But many people do use them.
Horses make me a better person.