The local currency is British Pounds. You should be able to get this at your local bank. The request may take some time, so plan ahead.
Your credit card should work. Note that some credit cards have a fee for international transactions.
Unless you have an international phone plan, your phone will not work. You could communicate through apps that use wifi, such as Facebook Messenger.
No! We will take care of renting cars to get from Edinburgh to the Highlands. If you wish to have your own car, you are welcome to rent one yourself.