A well in our area (east central IA) is 12-15k. Ours is 400ft deep, and a large industrial user may need to go 2000ft. We’re not far from southern WI, so the OP’s price estimate does not shock me at all.
OP, given that water supply is critical to the property’s intended use, I really do think it’s scary to buy it without knowing for sure you’ll be able to get water. States typically require well logs to be submitted for an new wells dug, so at least scan the state database for nearby wells, to see how deep they had to go for acceptable water. Here’s WI groundwater data page. that will let you search by county.
A bank loan for 15k would not be onerous to pay off-- I’d strongly consider biting the bullet and getting a well put in before winter. If nothing else, it may determine fencing and home layouts-- based on digging results, the well may not end up in the most convenient place.
You may want to have a geothermal tube dug under your water troughs, so you are not reliant upon electric power to keep your troughs from icing up. For remote horsekeeping, any added layer of resiliency in the event of foul weather would be helpful.
Finally, a winter where you park the horses a few hours away, somewhere that is set up for boarding, would be worth more than being able to ride for a few months.