Rushing depends on soooooo many things. She could be rushing because physically something is not right with her (something hurts, or physically she is not built to jump correctly) and so she has learnt that it is easier or less painful to jump quick. She could be rushing because she was pushed too quickly through her training or trained improperly. She could be rushing just because she feels it's more fun (OTTB's are notorious for it, as are any hot-bloods).
The main thing is you need to determine, is she happy when she is jumping? Does she want to jump? Have someone watch her jumping - what does her body language say? If she is eager and happy to jump than likely it isn't a physical issue, if she's eager but when she gets to the jump looks upset or unhappy she may have something wrong. Make sure the horse is vet checked if you do decide you are going to buy her, that way if it doesn't work out, you know that you can re-sell her to someone who wants to do something else with her.
The biggest thing I can recommend if you think the horse is eager and are sure there is nothing physically wrong with her, is to go back to basics, right away. Spend 3-4 months on basic training on the flat only. You need to build trust and a respectful relationship with the horse first (trust is a huge factor in jumping), and you also need to reschool the foundations of her training - walk trot canter, suppleness, and also very important is muscle, balance, and straightness. The horse has to be moving off a solid base before it can be asked to jump straight, quiet, and in good technique. Poll-work, hill work, suppling exercises, teaching the horse to work from the hind-end for 3-4 months. Then you can slowly build back up to jumping - first by adding jumping wings to the sides of ground poles, then with small x's, then small verticals.
Rushing isn't ever a quick fix. It can take 6 months to 3 years in my experience, but never less than 6 months. The issue is underlying - either in training, fitness, or in physical ability. It can also take that long for you to build a true relationship with the horse in order to gain the trust required to ask it to do something so unnatural (jumping).
With horses I've dealt with, keeping the fences below 3' for over a year has been necessary. And as soon as the rushing starts again you have to go back a step, two steps, even back to just poles. Height isn't the issue, it's the underlying. But find out what the underlying issue is before you buy, and vet check if you do!
Hope that helps! Rushing is a very hard thing to overcome, but it can certainly be done with the right horse and the right amount of time and discipline!