I like the sound of that thinking of the HP2561, is there anything else I need to get straight away
I assume you meant HP2551. You shouldn't need much of anything else, maybe a couple of Energizer Lithium AA batteries for the outdoor array. It is a complete system with console including Wi-Fi for publishing to Weather Underground and Ecowitt, sensors for rain, wind, solar radiation and UV, barometer, indoor and outdoor temp/humidity. Depending on how your current system is installed and mounted, you may need to reconfigure or replace the mounting system. Make sure you order the system with the correct frequency for your location.
EDIT: Maybe you did mean HP2561? I see that is a model that includes a newer console that has the barometer, indoor temp/humidity sensors integrated into the console instead of in a separate unit. Personally, I prefer the older design of the HP2551 console where the indoor sensors are separate from the console. For me the location of the console isn't necessarily the best location for the sensors. Also if the sensors are external to the console and fail, it is fairly inexpensive to replace them, but replacing the console is expensive. ... But that is a decision you will need to make for yourself.