Which default currency should I choose

Which default currency should I choose

First of all, I am considering to convert from Invoice to Books. Are there any feature I will miss in Books that I have in Invoice?

Second. I have a hard time choosing which currency to use as default. All (95%) of all my sales are in NOK. All (80%) of my purchases are in EUR. Still I do quote my offers in Euro, but final invoice and payments are done in NOK.

So what currency should I use as default? The company is registered in Norway, and tax reports etc must be in NOK. Bank accounts are NOK, etc. Still I make offers in EUR and talk Euro with clients all the way till the final Invoice and payments.

Need advice as I understand changing currency is a one way no return.. 

Third question. Possible to integrate with other banks easily?