Hi
If your keeping the car, you cant go wrong with replacing the battery
I bought my battery from the the battery mega store, it was a agm battery and its been great and the prices are also great https://www.batterymegastore.co.uk/
batteries on these cars cause all kind of issues, especially if its a diesel, they take so much out on start up
also some cars have two batteries, maybe be worth getting both checked
theres no guarantee a new battery will cure it, but its the best place to start and cures most problems like this
cheers
Joe