These are my personal thoughts from my experiences and in no way represent the forum.
I wouldn't write them off completely yet. I would draw up 2 lists, pros and cons. Thinking of things like, sludge doesn't necessarily mean she will get stones. Sludge can be peed out and on a good diet won't necessarily return. These girls sound like they have been to a good vet for a thorough check, you won't get that at many rescues and certainly not with a private rehome.
Do you want to take on piggies of this age? I took on a 4 year old boar two years ago, he is now 6 but it is strange when I think that because he feels new to me! I don't regret having him and am very glad I could give him a home.
Can you cope with 3 piggies, is your cage big enough for 3? Do you have room for a 2nd cage if the bond fails? Would you be happy with side by side living? Can you return them to the rescue if the bond doesn't work out. Older sows can be difficult to bond, especially as they are already in a pair.
Look into other rescues, how far would you travel? There are other rescues not on the forum approved list, they are sometimes not on the list because they haven't asked to be not because they are bad. Are there any general animal rescues that also have guinea pigs? There is no shame in not going the rescue route if there is no suitable sow available to you, whilst we would all love to rescue it's not always possible the happiness of your single sow is also important. I've been on a rescue waiting list for a friend for Bertie for over a year now, we have a very narrow chance of finding what we need in rescue with our difficult boar (9 failed bonds). He's okay with Duck as a bar buddy for now but Duck's recent illness has made me think. It looks like we will be going down the breeder route and trying a baby.
Somewhere on the forum there is a guide on finding piggies I will link it if I can find it!
There's a lot to think about. I wish you good luck whatever you decide.