Well it would be hard to judge without knowing anything more, but shes kind of getting up there in years. You would definitely need to have her vet checked to make sure they aren't selling her because shes lame or has some other problems. My horse is 20 and still acts like he will live forever! So her being 18 might not be that big of a deal. And older, experienced horses always make good first horses. Just make sure that you aren't overpaying.. remember that she probably only has like 5 years left before she is going to start slowing down.. or less! (Depending on the breed/ care. I don't kno much about that breed.) Also, consider what you will be doing with her. Jumping is harder on old joints than trail riding.
And light on her cues probably just means that she is very responsive. That would be fine for you as long as you could adjust to a horse that didn't need to be kicked to get moving! (which I'm sure you could!) The best thing to do would be to bring your instructor or an experienced friend to look at the horse with you.