I am not familiar with "hydroponic fodder", and whether that can be a substitute for hay, and if it's cheaper.
Whatever you do, consider that like a car, you must PLAN for breakdowns. Plan for vet calls, becuase they WILL happen.
Find out the cost of farrier, becuase so many OTTBs require shoes. That's every 7 weeks or so.
The shots, worming, and floating the teeth, cleaning sheaths.
Can you ask folks at your barn about these costs? You'd then have to amortize them over the 12 months of the year, add in the monthly board and feed.
Then, now you can go price the tack.