Hi Ian
Welcome to the Forum
The place to start is www.realoem.com it is a BMW online parts list just put the last 7 Digits of your Vin number into the search box it will call up your Model. Then search the sections you need to look at, there are small exploded drawings that should help with how and where things fit. Find the item double click the part number and it will show a list of any alternate part numbers and models that part was used in (very useful if sourcing parts.
There are some very tidy Android head unit upgrades available which are pretty straight forward to fit just take your time. The arm rest if you use a Vin Decoder you should be able to find the OEM trim colour then Google is your friend to find parts as you will have a part number from realoem
Good luck ENJOY your BMW
Dave