OP this is kind of adjacent to what I do for a living- I’m a lender and used to do a lot of horse properties. I know you’re paying cash, but I’m happy to discuss in more detail privately since I have been through the process with several clients. The flood zone concern may or may not be an issue at all. You will want/need an elevation survey to ensure that the portions of the property in the floodway do not contain buildings etc and have an idea of how that water table is going to affect your drainage, but I find non rural-savvy agents and lenders often pull the flood services that say yes/no as far as the property being in a flood zone without taking into account on a 50 acre property, that might be one acre of it or all 50 and those are two very different propositions.
You are almost certainly better off finding something with most of what you want and renovating, since what already is there has the benefit of being built with yesteryear’s labor and material costs. Right now there is such a shortage of skilled labor that building anything is a tough proposition and this project size is basically no mans land where you would need contractors with the skill set to put up large commercial buildings where your project is competing for the same labor pool with much bigger and more lucrative ones.
I will also note just because you don’t need a house, you may have better luck with looking at houses with enough land and appropriate zoning. For resale, there are more lending options available for residential properties with horse facilities than a strictly commercial facility, and you may find that staffing appropriately is a lot easier when you can provide housing even if you don’t plan to live on property.
As much as you’re going to hate to hear this, I think your best bet is driving around where you want to be and knocking on doors or having an agent do the same. The kinds of properties you want are prime for development and developers have full time staff looking for these and doing just that, so what hits the market is whatever they passed up in some areas.