I don't think $750/mo is outrageous. The area I am in now that would be pricey, but in my hometown in North New Jersey, that price for a horse like this would be incredibly reasonable! I would definitely find out about vet/farrier fees. Also, does the lease fee include lessons? As in, are any of your 18 rides per month in lessons, or are they all simple flat rides by yourself? If it includes lessons, then that's a pretty awesome deal!
She looks very cute and sweet, with a kind eye and very good form. I think in the video, the rider is not giving her the right amount of release, so she is carrying her head a little high and jumping slightly flat. That being said, she is jumping the height beautifully, and I'm sure with a little release she would be even more athletic and able! She doesn't look strong or out of control, and seems like a wonderful horse to introduce anyone to jumpers. I say go for it!!