It's most certainly not unrealistic. I am a 'advanced beginner' with lots of self teaching. I was taught to ride on calm lessons horses, but never taught to handle a hyper/dangerous/pushy one. I now have a rescue, a flightly three year old, and of course my pushy nine year (who is pushy no more!).
Find a reliable, experienced trainer first before taking in a PMU. Expect her to be flighty, untouchable, difficult, frustrating, and maybe even down right mean. Pretend that she's a three year old never-touched Mustang with an attitude. This is exactly what I expected from Sam when I first got him, he turned out to be the exact opposite, but you never know.
Then, just take it from there. Go real slow, gentle. Don't be afraid to ask for help even with what may seem like a stupid question. I'm gaining my three years trust by just BEING with him, giving him handfuls of grass, holding his grain bucket. Even saying Hi to him when I bring him his grain, calling to him when I'm in his paddock.
Sometimes the simplest thing can go a long way. If you have the patience and confidence (after a OTTB you probably do O.O) you can end up with a friend for life.
Even if you don't break her yourself, you can always take her to a reliable trainer to work with her for 30 days, or even 60 days, then go from there. I'm planning to do this with Red, as I don't feel that I am ready to 'break' a horse by myself yet.
Good luck with whatever you decide. I would love to have a PMU someday myself. Rescue horses can be very, very rewarding. And sometimes...annoying, silly, frustrating, irritating, down right make you wonder what that horse was thinking...
Wait! I'll fix it....