Okay, first of all, intro dressage is simply walk trot. I don't want to offend you by saying this or anything, but any horse with proper basics in flat-work could bring home endless ribbons, firsts, reserves, and champion ribbons as long as they are relaxed, consistent, and obedient.
Secondly, keep in mind she's already 13 years old. She is likely pretty set in her training and be aware that she may be more cautious and a little more nervous about something completely new being introduced to her. That's not to say an old horse can't learn new things, just keep it in mind.
Thirdly, if you can, see if the current owner will let you take her somewhere nearby to go cross country schooling. You need to get her out and see if she'll be okay jumping out in a big field by herself over solid fences. Even if you're just jumping over the tiny 2'0" amoeba jumps, you'll be able to get an idea of how she handles cross country, especially if you have the opportunity to string multiple jumps together and create a mini "course" of sorts.
Also, I know you said she hasn't done much jumping in general, but in an arena ask her to jump some things that are more than just 2'3". Most warmbloods have an aptitude for jumping (they're bred for it!) but don't actually enjoy it once the jumps start having more substance.
Of course, get a thorough pre-purchase exam and all that jazz. Call up the horse's current vet and ask for records of any previous injuries or ailments. Plenty of warmbloods compete in eventing nowadays, and I'm sure this mare would be fine taking you around the lower levels.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."