A horse that won't move forward is a red flag, in my opinion. Lazy? Maybe. But just as likely, she doesn't respect the rider and feels she is dominant over him. When you can't get a horse to move forward and you persist, (spurs, crop, etc.) you don't necessarily get what you want. Sounds like she needs an experienced rider w/ good command of the reins and use of his/her legs.
If you do decide to take this on I would start with gaining respect from the ground. You say she is nice on the ground, but that is not transferring to the saddle. Is she nice on the ground because you are petting her, feeding her etc? Or is she nice on the ground in the round pen, on the lunge line, and when she is being asked to work, turn, stop, back up, etc. ?