RE: integrate xe.com API with Microsoft Dynamics NAV 2013R2
Basically, you will need to do one-time integration with XE, using their web service.
The question is, would this data be correct for your purposes. If you implement your solution in one country, it is enough to make an integration with this country's Central Bank online service - you will have all currency rates, related to this country's currency. And, this will be a free service :-) after you will setup similar API connection with Central Bank's service.
Using XE is reasonable if you go to multiple countries with same solution, your will save on integration costs in each country - is it your case? If not, I would just recommend integration with your Central Bank (if it is not even part of your localization).