Range Rover Battery

Help pls have recently bought a P38 range rover diesel 2000 model have charged the battery and done the eka code and resynched the key fob have done pretty much everything that i personally can do apart from spend huge amounts of cash on diagnostics and such but still refuses to start or turn over there is a clicking sound followed by the lights flashing and the horn beeping it is also throwing up quite a few warning messages some of which have been disabled have scoured the internet for ideas as to what to do next i have found out from quite a few sites that it could be a simple thing such as the battery not providing enough power now my question is which battery do i need to replace the old one with as the old one has no sticker so have no idea of the cca and such any help would be much appreciated thank you in advance

Welcome, you need the biggest you can fit in the space.