I am going to go against the grain here...
I have been advised by vets in my area that breeding a maiden mare over the age of 10 is not a good thing to do.
Usually, mares that have had a competitive career in their lives are much more prone to endometriotis, especially older ones, this means that she will have a difficult time catching, and the foal may be at risk during pregnancy. Treating endometriosis during pregnancy is also costly.
Also, older mares whos bodies have not carried a foal before are not used to it, and it can be hard for them to adapt. Just think of people (Sarah Palin comes to mind). Women over 40, regardless of previous child-bearing history, are advised against getting pregnant because it can be very detrimental to the child.
I would also advise you to read the "Breeding Gone Wrong" thread. Especially because your mare is older, if there is a problem during birth she may not make it.
Please think about your decision! If you do not have a lot of money to spare then you may not be ready to breed. In my experience it can cost well over $10,000 just to get a mare pregnant.