At certain times, I bring in more income from rabbit poop than rabbits. I clean out the barn roughly twice a year, spring and fall, and shovel the poop back into feed bags (as my son observed, "New Look, Same Great Product!").

This fall I filled 67 bags, which equates, very roughly, to 3,350lbs of manure. Some of this I sold, some we used in our own gardens.
So, I am obviously in favor of producing and sharing rabbit-generated fertilizer. However...
We have 32 cages, an assortment of large and small breeds, and we do breed them throughout most of the year. It is a fair bit of work to care for that many rabbits, shared by two daughters and myself. We use pellets during most of the year; if you are planning to feed them only forage, it will be that much more work to collect and balance nutrition for them (though if you're not breeding, that will be simpler than trying feed milking does and growing kits). Rabbits do NOT like change, especially as they age, so the best plan will include a way to keep what you're feeding them fairly consistent.
There is a pretty good book about feeding natural sourced-feeds called "Beyond the Pellet" by Craven and Warden that lays out a lot of the basic considerations:
https://www.amazon.com/Beyond-Pelle...hvlocphy=&hvtargid=pla-4584070153170486&psc=1
Another thing to be aware of is that getting clean, dry manure will require a bit of effort and planning. Our rabbits are in wire cages, from which all the waste falls on the barn floor. Thus it ends up mixed with hay, fur, dropped pellets, wood shavings (from nest boxes, which you won't have) and of course urine. It's not "clean!" That is just fine for compost, but if your buyer wants "dried rabbit poop" you'll probably need to set up a system to keep the manure clean and separate from the urine, which is what smells. On the other hand, urine is an outstanding fertilizer in its own right, so your buyer might be happy with a mix, or you might find an outlet for that if you collect it separately.
As far as your age and land, you'll have to be the judge of the first. If you have an acre, a good barn could house that many rabbits, but you'll have to figure what you could grow and how much if you're hoping to feed forage. Some forage components, like willow, clover and and comfrey, grow well without a lot of TLC. If you're going to have to be haying, in my mind that might put the time and labor investment up too high. If you can find a source for some of their food (barter?) that might bring it back into a reasonable range.
Good luck and God bless!