Buying a car/ more nie advice please (Balearic Islands)

Hi
I posted a little while back about getting an NIE number for a British citizen. Unfortunately have not had chance to sort out whilst i was over back over in June.

I am wanting to buy a car and am told I need NIE to do this too.
In order to get an NIE, one of the requirements to submit is having a letter as to the reason for wanting an NIE. What does the office accept and how can I get such letter? Sorry for being ignorant about it, just never done this before.

Many thanks

Will