So you've been paying for the privilege of paying all of his expenses, including board....
I wouldn't offer $250 for this horse, much less pay $2500. The tumor in his mouth will eventually cause difficulty if it continues to grow. Large Vet Bill.
The tumors on his penis also have the potential to cause problems with his being able to urinate, resulting in bladder infections and possible kidney damage. Huge Vet Bills.
I'm betting the vet check was also the 1 year adoption check -- which is why she was unwilling to sell before. Note: My rescue wasn't officially 'mine' for a year.
She's suddenly willing to sell after the vet discloses the information above; she's seeing the same thing I am. Vet Bills. She's also seeing YOU. Someone with hearts in the her eyes and cash in her wallet and thinks she can off-load a problem and make a profit at the same time.
I'd drop the lease, let the BO start picking up the costs, etc. and see if the price comes down. At the same time, I'd offer to take the horse off her hands if, a) she can prove she's met the rescue/adoption time rules and, b) if she'll take what the horse is actually worth - which is nothing.
I'm sorry if it sounds like I'm being cruel, but a horse of his age and condition probably isn't going to be usably sound for more than 5 years - and you'll be lucky to get that. After that, you have the expenses of an aging pasture ornament. Board, feed, vet, farrier, etc.
I know that you love this horse, but the BO doesn't. She just wants to make back whatever monies she's already spent. That isn't your problem. You need to look at what the future will cost. That $1000-1500 you say you can afford is best held onto for this one's vet bills, or a different horse.