Sorry to hear about Lucie's torn ACL (CCL). My 9.8 lb., 10-11 year old chihuahua, Rascal, is now day 4 post-op from bilateral ACL tears. The rehab is harder to deal with than the surgery itself. After his right leg tear was diagnosed (he had been having more and more problems jumping and stair climbing), we were trying to keep him from jumping on the couch and placed a gate in front of the stairs. Things were going well until he was determined to come upstairs to see me and jumped the gate, thereby tearing the left ACL. Due to his size, the orthopedic surgeon determined the older "fishing line" repair would be best. it was my decision to do both knees at once since it would mean one anesthesia and one rehab period. The price was around $2500 for one and $3400 for two at once, so that was a factor also.
Keeping your pet's activities as limited as they need to be to allow a conservative medical approach to stabilizing the knee is very difficult and requires constant vigilance for quite a few months. Having to keep our Rascal in a crate, which he HATES, has been sad and exhausting, but after 2 weeks he'll be allowed limited walks and start physical therapy.
I hope whatever you decide, Lucie gets to feeling better real soon.