Re: Looking at buying a horse with a cataract in his eye.
I nearly bought a horse with cataracts many years ago, she was a lovely grade b showjumping mare, but when they found the cataracts her owners reduced the price from £6000 to £700, I still wanted her but as I was working as a groom at the time, I decided against it as if they got worse (which the vet said they were likely too) I would have very large vet bills, at the time I couldn't afford it, I was lucky that the new owners kept her at our yard for a while and I still got to ride her but its a personal choice, she was retired from Showjumping too.
I found this article that might help you, if you google "equine cataracts" theres lots of info out there to help you choice what to do:
but I agree with the insurance bit, if there is an existing condition most insurance companies won't insure or will for a large price and conditions. My horse got a cough and they tried to take off his respiratory system off my policy!! they really don't like to pay out so might be worth getting a few quotes to see.
good luck with whatever you do, and a second vetting it always worth it, especially as the cataract showed within a year.