My concern with Bonnie is that you said that she may be unable to cope mentally with the stresses of agility. Border collies can be highly strung and is a temperament best not bred from particularly as you dont know her lineage. My friend has a very highly strung BC he rescued from the pound, but being an experienced BC owner has manged to deal with it.
This is where knowledge of your dogs lines comes in handy, it gives you avery good picture of what may be as a consequence of breeding.
So a resilient temperament is good - a dog that recovers very quickly from an unexpected event and is very stable in the event of pressure, so copes well in stressful situations. I like a dog that is able to cope quickly with the unexpected or a certain amount of pressure. My best dogs have very confident, extroverted personalities.
The other thing you need to prepare yourself for is if anything were to go wrong with the pregnancy. A breeder friend of mine recently lost one of her best dogs when the uterus ruptured. It cost her thousands of dollars and even with the best vet care, the bitch died. She then had to hand rear the pups. There are sometimes complications which may require a C-section. You also need to have money aside for pre natal xrays and care.
So the question you need to consider is are you prepared for the worst case scenarios. It may never happen but I know a number of situations where it has, so being prepared is essential.