I think just based on the fact that you are asking if you should breed her, that you shouldn't breed her. If you were really planning on it, you wouldn't need advice from other people.
Now for the horse. She looks like she maintains her weight well for a 21 year old. She has produced at least one foal before which is good. If she was a maiden, I'd say now way to breeding her. If she is healthy and up to date on everything then yes, you could breed her.
Now, here are some questions.
Her pedigree states that she has some good horses in her. Which is good. But, how is her conformation? Some good pictures of her, full body shots (including legs and hooves), back end shots, front end shots. Has she been tested for genetic diseases? If she carries Impressive in her pedigree, has she been tested for HYPP. She looks QH, correct? Has she been tested for other diseases that Quarter Horses are known to carry?
What does she have that is breeding quality? Does she have a nice temperament and work ethic? It's not all about pedigree's and conformation, you have to have a good disposition as well. Does she have a show record?
What type of stallion are you looking to breed her to? If it's just a "decent" quality stallion for $400 off of craigslist, I wouldn't even consider it. You should have the nicest possible stallion that you can afford. I wouldn't even go under the $500 mark unless the horse has a show record and the breeders are just trying to keep the price low due to the economy/younger horse just starting out but has exceptional potential for a stud. (The Great White Hope is a prime example. His stud fee is $400 right now but he has a good pedigree, I like the way he looks, and he has a show record)
Ontop of all of that...Can YOU afford it?
Can you afford the stud fee, booking fee, AI fees, vet fees, ultrasound fees, feed etc? A older horse like this is probably going to require extra care while pregnant. You MUST ultrasound her after 18 days to confirm she is in foal and to check for twins. Twins are deadly for a younger horse, let alone one much much older. And it's best to know if she's bred or not so you can plan the next year accordingly.
She'll require extra feed. Maybe special grain/supplements. She looks to be in decent shape, but I don't know how old those pictures are. You'll need to keep her in top condition so she'll be able to make it through labor without many problems. A good exercise program is best and will require more work on your part.
Someone MUST be home with her while she foals. Since she's older, she's more likely to have problems while in labor. If you can't be home to keep an eye on her, don't breed her. You could lose her very easily.
Lastly, are you prepared to lose her if it comes to that? And are you able to properly care for an orphaned foal? Breeding a mare is a big commitment no matter what age the horse is. It's best your prepared for what could happen, not saying it will, but stuff goes wrong and it's best to know beforehand rather then get in over your head.