Hi!
It is great you want to your best for Bella.
Guinea pigs that have always lived alone or that have made bad experiences can be very fear-aggressive with other guinea pigs.
The best way of finding out what is the matter with her and how she is with other piggies, even a pair next door is dating at a good rescue that offers this service (sadly many RSPCA branches won't do it or are not all that experienced with guinea pigs. Especially older sows, even those that have previously lived with another piggy or even in a group for all their lives can be very tricky to rebond, as I know from my own experiences. A few piggies can be so fearful that they go on the attack even through separating bars.
I would NOT recommend to get a guinea pig on spec.
Here is our list of carefully vetted rescues that we can vouch for Bella and you being in safe hands:
Recommended Guinea Pig Rescues
This is an article about single guinea pigs in their various situations with their particular challenges that I have written for Guinea Pig Magazine earlier this year. You may find it interesting and helpful:
Single Guinea Pigs - Challenges and Responsibilities