I really haven't had to deal with true canker, aside from one horse that was recently diagnosed just before I went on maternity leave.
This is one area I'm glad to say I don't have much dealing with.
The one case I came across that I mentioned, is a young draft cross that was severly mal-nourished and had a multitude of hoof problems as well as other issues in his body. He's doing well, his feet just stay rotten, though. With constant attention, he's not lame. I'm currently not working on him, since I'm 8 months pregnant. I believe the owner has great luck with frequent apple cider vinegar soaks (about once a week) and he's staying comfortable and in work (ground work /manners training) . When she skips the soaks, he starts to have trouble again (that's what I meant about his feet staying rotten since it always comes back). But this horse was just pulled from bad circumstances last summer, so it's not been quite a year, either. (and yes, this is all with a vet's attention, as well)
This horse had in the beginning, been foundered, as well, and I believe he's an extreme case of ANYTHING you can think of from skin problems to loose stools. Metabolic issues I'm sure aren't helping anything.
I think it's a good sign that I haven't really HAD to deal with canker, and I'm sure it's not any more or less common here in KS than anywhere else. So I would think with proper care it can be managed if not cured.
A good dry (as possible) environment helps keep a lot of hoof problems away. Exercise is good medicine for hooves if the horse is not in severe pain by stimulating circulation and blood is what brings antibodies and nutrition to damaged tissues. Diet, low sugar, high fiber is always healthier than high grain diets, so that could be a factor, as well. Also, none of my clients shoe their horses, since I'm a "barefoot trimmer" . Maybe this is why I don't have to deal with canker? Not sure. I just know that once the horses are barefoot and adjusted, there are fewer and fewer problems I see of any kind.
That being said, as far as buying a horse with a problem you aren't sure about is a very personal desicion. Only you know what you are prepared to accept and remember there is a chance he'll be an expensive horse to own with little riding time. But, once he's recovered from surgeries he may be fine, but are you willing to wait out the recovery time?
Me, personally, I'm not afraid to take on a horse with hoof issues, but I get free hoof care, so to speak. Paying for the upkeep could be daunting for anybody, and I can't decide for you.