Hi Mitchy,
You have had lots of advice there! Have you come to any decision?
It sounds as though you are an experienced horsewoman and have a number of horses already, and probably all the skills you would need to work with an ex-racehorse.
BooBoo raised lots of important considerations upthread, but the bottom line is that each horse is different and each trainer's routine is different, so each horse's life experience will vary.
I have a racehorse that has just come home to me with a pelvis injury. This is his second serious injury, both requiring prolonged periods of box rest. We had him living out 24/7 before he went into training and never had any trouble with injuries or illness. Despite being cooped up for the last 7 weeks he is like a LAMB in the stable, the sweetest horse you could meet.
The yard where he was trained did plenty of roadwork with him, most racehorses will be well used to the road and hacking as it is so good for fittening them.
They also spent the first month working with him on the lunge, so he has some experience of this type of work, but not a whole lot.
I am hoping to ride him for the first time this summer , am nervous and excited. He is my first horse and I wouldn't say I am as experienced as you are. The trainer says he is as 'quiet as an ass' (despite winning two point-to-points) so we'll soon see...
I would imagine that working with any ex-racehorse would be an exciting and challenging prospect, but one that demands patience.
My TB loves to live out. During the worst weather we rug him up and just bring him in at night.
Regarding competing an ex-racehorse, if he has actually raced, you could have a long road ahead before he is showring-ready. My horse used to trot right into the horsebox. Clearly going racing has affected him. He is now a terror to load and unload. Then, I imagine their experience of crowds, loudspeaker, other horses in a competitive situation could stir up quite a bit of nervous energy and excitement.
I suppose, to sum up, rehabilitating an ex-racehorse could be a very rewarding journey, and whether to acquire one direct from a trainer or from a centre would depend on how much of the groundwork you would like to do yourself with the horse.
Good luck!