You will have considerably more land that you need for both grazing a horse and producing enough hay. As already pointed out though, if you plan to make your own hay there's a large outlay in cash for equipment (you add on a truck if you don't have one or a trailer....you really don't want to hand carry that many square bales out of a field and you couldn't carry a round bale which will also call for at least a fork attachment for the tractor of some other means of lifting it if you don't plan to just leave it where it is).
Depending on how much land you set aside for hay, how productive the land is and how cooperative the weather is you have the potential to produce much more hay that you would need. So you could sell some to help with recouping some of the cost, but don't expect for it to pay for itself any time soon (you'll figure that out when you look at the cost of just a good baler, let alone all rest)
I'd recommend getting a pasture mate for your horse. Doesn't have to be another equine (there are less expensive options). Horses are social, herd animals. They feel more comfortable and secure with another herd animal so even a goat will help meet that need.
You'll have enough land that you'll be able to practice a great rotation system that will help keep your animals and pastures healthy.
Keep in mind that even two horses couldn't hope to eat the amount of grass or hay that could come with that much land. Also, horses, unlike all the other domestic grazing animals are best equipped for eating right down to the dirt (because they have both upper and lower teeth not just lower teeth so they can snip it right at the dirt) which is why they have the ability to over graze an area quicker than any other livestock.
Partition your pasture area off. Preferably 3 or 4 sections (about 2 acres each if you just have the one horse). Put the horse on one are until the grass is grazed to less than a couple of inches long then rotate to another sections. Try to keep the grazing you're moving to at about 4-6" long (so you might need to cut them if they get too long between rotations) since you don't want your horse on tall lush grass (want to avoid grass founder as well as other things).
This rotation process will do two things for you.
1. It will keep you're pastures healthy by being grazed without being over grazed and fertile (horses doing a lot of pooping).
2. If you're rotating about every month you'll quickly have no worms to worry about and get just have the Vet do about 3 FEC at certain times during the year (my next one due next month). My horses don't get wormed and generally run between an FEC of 0 - 100 (only had 100 on one of them once). Any count under 150 is considered to low to worry about worming. Ultimately it can result in your horse becoming resistant to the worms (which is much better than the worms becoming resistant to the medication).
I know TMI
. Sorry about that. Blame it on all the years I spent in my youth raising stock
You have the makings for having what some call a nice "gentleman's farm"
. I hope it all works out well for you.