May I ask how many you intend taking on at one time?
Because fair enough your plans have changed which is why you asked advice, but if you are only doing one and you now plan to keep them for 3 to 6 months before training them and then only putting in how many months on training you are looking at close to a year before you are able to sell them on and what kind of profit do you expect to make? Feeding that horse 2 to 3 meals a day, buying hay (or teff and lucern) farrier bills, teeth bills, possible vets bills and unforeseen accidents, innoculations, ulcer treatment etc etc etc is going to cost you more than you could EVER hope to make. Should you be taking on more than one it (A) means you have double the work to do and (B) double the bills. Or triple or more depending on how many you have at a time. Yes, you might breakeven when you sell the horse or even make a profit based on the fact that you got the horse for free but that money is just going to cover all the expenses you have incured along the way.
In my personal experience these horses also need to be kept in at night to hold their condition. They let down and some let down worse than others. They arent used to being out on grass all day and possibly night. It's something they need to be weaned into. It's a huge culture shock for an animal that has spent 22 hours a day in a stall to then be turned out to grass and expected to socialise and just fit in to a new environment. Some don't WANT to go out to graze because they are so used to being in. Others will run a fence aimlessly and lose condition. Some just stand and stare. Every OTTB is going to deal with the transition differently.
Im sorry if im sounding negative but I really don't think you will ever make a business out of something like this. It sounds like a sinking hold of financial despair. If you were taking on a horse for personal use I would say go for it. And should you decide to continue do yourself a favour and read the book "Beyond the Track: Retraining the Thoroughbred from Racecourse to Riding Horse"
you can buy it off Amazon
.com it will really assist you with the basic reuirements an OTTB needs.