Malinda, your situation is so much like mine right now. You had 11, and we currently have 11...your one had throat tumor, and our Buffy has mouth cancer (around and under her tongue). She is hanging in there ok so far, but last couple days I notice she sleeps just a bit more. But she so loves being brushed and sat with, purrs and headbutts, so as long as she does that, I can't make that decision now. But we may be letting her go at some point in the not too distant future. She still runs away from my syringing her, which is good. When she just sits, it will be time. But we'll relish the time with her until then. But...that said, when she's gone we'll have 10. And we too, have mostly seniors. Our youngest though is 6 (Parker..actually thought he was older but looked him up in my cat birthday book) his mom who's 8+, and then they go from 10 to nearing 17.
How fun that you will be getting two youngsters again, and one a baby!. You'll have to give us an update on how they are as they acclimate. And pictures too!
Good luck on your trip and your new experience!