Sounds like a plan
Some RSPCAs spay/neuter, but others don't, so make sure you check. If you're volunteering there then likely you'll be able to work out which bunny you want and which one clicks with you because you'll already know them, which will be a bonus
Shame you're not down here and not looking for a bunny now, where I volunteer (at my local RSPCA) we have the most amazingly humanised and soft bunny. She would probably be your perfect nightmare as a house rabbit
(only a nightmare because I feel she might be a destructo bun, but she is just so pepole friendly she is incredible). There will be plenty of rabbits around that will fit with you hopefully
Also, well done for doing your research before getting any bunny
Whoever you choose will be very lucky to come into your house because you have several months to get everything perfect before any bunny arrives in your home