I would checkout
John Houck's articles on course design. He's generally considered the leading expert in course design nationwide. There are plenty of other people who have created amazing courses, but John's the only one I know of who makes a living at it.
Standard logic is that you can make a decent course with about 1 acre per hole. What should really dictate your buying process is thinking about what KIND of course you are creating and WHY you are creating it.
If it will be a place you are thinking of hosting big tourneys and stuff, then you should also
keep in mind PARKING space and CAMPING space. Where are you going to put people?
If you are going to be caring for the course by yourself, think about how you are going to MOW fairways (time consuming and expensive at some places). Some pieces of land have very little mowing/clearing. Others are a nightmare and take incredible amounts of time and money to keep clean, even if they are awesome when they are freshly mowed.
One of the beautiful things about disc golf is that it often works best in land that is frustrating for other purposes. Weird elevation changes and odd streams and mixed terrain are less than ideal for farming or raising livestock or even for building development. But they are IDEAL for a disc golf course.
My favorite private courses in the country are the pair in Paw Paw, WV. I don't know how much land they have out there total. I know the Woodshed course is on ~25 acres. I'd guess the other course is about the same. And those are amazing courses with plenty of parking and camping space. It's also where the course owner and his family live full time, so the house, barn, etc... are there.
It might be worth making it to some various private courses around the country when you have a chance and seeing what has been done well. Paw Paw, WV, is my favorite, but The Grange in Spotsylvania, VA, is also excellent. And then, of course, you've got Pyramids and Maple Hill up in MA... The two Houck courses down in Wimberley (currently closed temporarily) are also worth a visit if you can get the chance to go. Moody's Red Rock outside of Austin is also worth a gander. Those few locations would get you started if you're looking for some ideas.
Are you looking to make money off the land as well as play disc golf, or not?
Hawk Hollow in VA, is another top course -- it's a cow farm year round (the cows have right of way on the holes if they get in your way) -- but an amazing course. Pyramids and some other private courses have areas of the course dedicated as Christmas tree farms -- the trees grow fast and are an interesting obstacle on a few holes, and help support the course in the off-season.
Also think about future expansion. Would you ever want more than 18 holes? Are you adjacent to other land that might be available for purchase in the future? The best private courses in the country offer 36 holes and camping and are ideal for the best big tourneys. If you're just looking for a place for you and a few friends to play, then this isn't a concern.
Are you planning on putting in a crazy hard SSA-70 course are are you just wanting something fun to play around on? Obviously, the shorter course takes less space. The 1 acre per hole standard applies mostly to courses that have a lower SSA (I'd guess 50 or less). Anything more than that is going to take more space if you want it to be safe and not have crossing fairways.
What all is realistic:
The Grange has two 18-hole courses -- one 75/25 in the woods (The Darkside), the other probably 40/60 in the woods (The Sunnyside). But it also has an 18 hole par-2 "Tiki Course" where everything ranges from ~90' - 150'. Putters only, really. Crazy fun rounds. Everything is strung up with Christmas lights and totally playable at night. High entertainment value and it will teach you how to use your putter.
Codorus (public course) in PA has a 9-hole course with mini-baskets for throwing disc golf minis. And it is an incredibly fun course -- challenging and really quick to play.
If I were buying a piece of land, I'd want to have the option to host tourneys, and would probably try to get a space that could accommodate at least 18 challenging holes, as well as a good alternative course -- like a Tiki or mini-basket course -- that would provide a quick, fun factor ideal for late nights or for beginners. With a decent piece of land, you should be able to put all that and some space for camping/parking in ~30 acres. More for a harder course or for more holes.
I hope this helps.