Cow hocked means the hocks point in towards each other. It's a fault that usually causes the horse to toe out as well. A small degree of cowhockedness is common, this mare's degree is not. She is cowhocked and toed out enough she will not travel straight behind, robbing her of even more power behind.
She is the opposite of sickle hocked. Sickle hocked means an exaggerated angle in joints of the hind limb, this mare is too straight (post legged).
As far as her neck, yes, I do think it's too short to be effective making the kind of balancing gestures a jumping horse needs to. If you use the old analysis tool of dividing the horse into thirds, her middle third is much longer than her hindquarter or her head and neck. If you look at the top photo side by side with a photo of a well proportioned horse, it becomes clearer.
Also, wasp waisted refers to the bottom line, not the top line. She is also weak in her loin coupling (top line). She is tucked up/wasp waisted in her bottom line, but that could bw a result of her immaturity and condition, not a permanent feature of her conformation.
She appears to be a sweet mare who might give decades of good service as a pleasure horse or light trail companion. She is not a good prospect for uses that require a high degree of athleticism. Whether or not she's a good choice for the OP depends on how important and how serious her competition goals are. If she wants to lark over a few crossrails in a ring and go to gymkhana play days in addition to pleasure riding, this mare is fine. If she wants to do more than that, I think she should keep looking.