How Much Should I Offer on This Horse?

Hi everyone. I'll try to keep this as short but detailed as possible.

A month ago I met a quarter horse at the barn I was leasing a horse at and fell in love with him instantly. He was/is a rescue horse who was about 250lbs under weight at the time and being used for lessons. I asked the owner if I could buy him that very day and she said she wasn't ready to sell him, but she let me do a full time lease on him.

For the past month I have been putting weight on him (paying for all of his supplements), paying full boarding fees, visiting with him daily, taking lunging and riding lessons with him. I'm just so in love with this horse. He is a good sound horse, but he does need some work, and really needs his topline built. He's also covered in scars from whatever crappy life he lived previously.

I was at the barn the day he got his teeth done, and the vet came out as well. The vet says he is in his late 20's, has a tumor in his mouth, a cluster of tumors on his penis, as well as a couple of tumors on his bum. The owner believes he is actually in his early 20's, but as a buyer, wouldn't I go by what the vet is telling me?

In any case, the owner wants to sell him right away now and she is asking $2,500.

I am really only prepared to pay between $1000-$1500 for him.

I'm very new at all of this so I was hoping to get some advice before I make a counter offer to her. I'd like to have some knowledge in my hands to say "he is worth xxx amount." I am wondering if my counter offer price of $1000-$1500 is reasonable given his condition and age?

I'm sorry if any of this is confusing, it's a weird situation but the owner (who is also the owner of my barn) Is a good gal and she does care about this horses well being, but I also don't want her pulling a fast one on me, assuming that my novice horse purchasing experience can be taken advantage of.

Thanks for any help!