Even though she's not a maiden it's been awhile since she last foaled so you may have a very hard time getting her pregnant, especially if you waited another year at the age of 20. One of my mares I've bred 3 times, last foal was 4 yrs ago and she is now 18. I asked my vet recently if I wanted to breed her again did he think she would conceive (history of being an easy breeder, one cover she's pregnant). He said to do a biopsy of her uterus and if it can back anything but a Class I or IIA not to bother. I decided to not waste my money or risk my mare.

I think with her being older and honestly not the best quality, I would say not to breed and buy yourself a nice foal.