I can understand your hesitation. I think your best bet will be a thorough pre-purchase exam by a vet that you trust. It is getting more and more common to see horses being competative up into their 30s. I would start with the PPE and move on from there. If she's clean on that, then there is really no reason why she wouldn't be suitable for what you would want. Being her age, it probably wouldn't hurt to keep her on a joint supplement as a preventative but past that, she should be just fine. Don't count her completely out. Go and give her a good ride and if she is just what you are looking for, then take that next step.