If you're really going to try this, I can not stress enough that you MUST start small. I know it is tempting to take in a lot of horses but you need to make sure that you can keep up with the amount you own, so take them in one or two at a time and give a good amount of time between adding more and STOP when you start to feel a slight stretch, don't even look at more unless you sell some that you have or they pass away. Really, it is SO important. I know we all think we can handle it but each horse needs to be able to be thoroughly checked on a very regular basis because problems can develop anytime. Make sure all of the horses can be handled to be checked, have feet checked and trimmed/shod, get shots, be dewormed, etc. and that the horses are all keeping weight well and fed seperately or extra if there is a problem (and that no horses are very overweight, either)
And make sure that there are no stallions! If you come into possession of a stallion, get him gelded right away and seperate from the others until he has been gelded..because you know random breeding like that is a major cause of the excess horses that wind up being neglected in the first place.
You may know all of this, but I just want to make sure. It is a very nice idea if done right. Good luck in your venture!