First- I have a 15yo TB gelding with stifle problems. I can tell you that they can be devestating. We went from competing at training level eventing possibly moving to prelim to retired and looking for a new horse in just 2 months. 28k is asking WAY too much- even if her breeding is great.
Second- The vet might not be able to find an actual problem now, but because of her confirmation, there will be one sooner or later.
Third- If you really love her and have to have her, be ready for a ton of work. I have to ride my horse 6 times a week to keep his stifle strong. I tried to give him a few days off, and we were back to slipping stifles. The vet said I shouldn't jump him any more and dressage is very hard on him too. He can't do smaller than 20 meter circles or be lunged because of the stress on his joints. Be ready for a ton of hill work and hours of trotting poles to get this stifle stronger.
Fourth- Sometimes the vet can inject the stifle, but other times they can't. I've heard that injecting the stifle is expensive and only lasts a handful of weeks. It's like a hock injection, but the horse has to be sedated. With my horse, injecting it won't help because the problem is a tight ligament that locks over the joint so they can sleep standing up. Since the joint is what gets injected, injections won't help with tight ligaments. My horse gets a shot of Depo Provera every 2 weeks to try and keep is muscles loose in his hind end. The farrier might be able to add wedged shoes to help with the angles of the joints in back. This is what we did with my horse and it helped a little bit, but not a ton.
Fifth- A weak stifle can lead to a slipping joint which is bone on bone rubbing. This can cause/ aggrivate arthritis and is painful for the horse. A slipping joint can lead to a locking joint which is very scary because a locked joint isn't far from blowing the joint. If you are riding the horse and it blows it's joint, the horse will fall. I've seen it happen at a show and it is terrifying and painful for both the horse and the rider. Most horses can't be worked after blowing a joint.
In my opinion, 7 is WAY to young to have stifle problems. I understand weakness, especially if the horse isn't worked much or correctly (you said she was fat). There is no way to tell if weakness will turn to problems or if you will be able to strengthen the joint, but her confirmation and the fact that it's hard to keep her cantering, leads me to believe that it will be a long term problem. I had a horse vetted back in April who was 5 and carried extra fluid in his stifle. The vet said it could be nothing, or it could be a problem- there was no way to tell. We walked away from that horse. Any other joint issues are fine, but the stifle is a deal breaker for a lot of people. Please, please, please think about the consequences in the worst case scenario. Are you ready to have a 7 year old EXPENSIVE horse that is unable to jump and show because of joint problems? I'm sure there is something that I'm forgetting, so if you have any questions- ask away. I've been through it all with my gelding and I can probably answer most of your questions. Really consider this- I'm not in a place to say, "Don't get this horse", but the stifle is a major concern- especially with such a young horse.