This has been very informative and will no doubt hold me in good stead over the next few weeks analising the information given. So, you don't need anymore from me. On that note I will continue to add my humble opinion.
STOP worrying first you have to find your horse. You have the land and feed available so build a smaller paddock 50 yrds square. That is to put him in if he eats to much, but first put the horse in that area or mow it down. This is a holding area he/she can't over eat so you have some control.
When you find the horse that steels your heart, and it will, we can see that coming. Take note of its present living conditions, is it free roaming, is it given suplements, what is the present owner doing for exercise, (I mean for the horse)
Note all the information and take the horse home. If the horse has been stabled and you are not going to, then make the adjustments to the horse slowley at first giving it a chance to get used to its new way of living. If its never been turned out to forrage then bring that change in, in stages so he does not over eat.
When my horse Stella arrived she had been wintered with a cover on so I continued that for the remainder of that winter, but not this winter she had to grow hair and be a horse. (she was not happy)
So go and find that horse, make changes to its life habbit slowley and all will be fine. On a final note a horse is like having a teenager in the house. It cause worry, it challenges your intructions and it will try and boss you, on a regular basis. Enjoy, they really do give in return.