Crystallin

We have a 1998 Outback Limited. Except for a minor oil leak it has been great. Driver’s door was left ajar overnight with keyless locking activated. Next day car would not start. Cabin light is dim, there is low power to heating system and windows, some of the dashboard lights but apparently not to radio. Sounds like the battery is low. I was surprised it drained overnight just from an open door but it is old (original). I plan to replace the battery but I am wondering if there will be any problems with the security system? Right now the transmission is locked in park. Security system light sometimes flashes fast and erratically. Sometimes there is no light but instead there is a clicking sound from the same area that starts slow and speeds up. Flashing light and clicking can be cleared by turning ignition key or hitting locks, but after a while starts again. I disconnected the battery to stop this. Will a new battery fix all this? Anyone been through this before? Any hints? Thanks.