There is already Statistics by currencies available from CustomerCard - no custom coding is needed.
As for credit limit, here the case is more complicated. Nevertheless you can define “default” currency for a customer, that does not explicitly mean only this one currency must be used, you can easily change it to any other or local currency, when creating sales docs. Therefore, by design credit limit is in LCY, allowing to deal with cases when customer has outstanding balance composed of several invoices each in a different currency.
If you still wish to set credit limit in customer’s “default” currency, a mod is required, and rather complicated one, as simple recalculating limit set in LCY & displaying additional field in some currency will be of no use, system will check against the standard field in LCY. To change this, more modifications must be done. Besides, FOREX rates change daily - a slight shift in rates may cause a, say, 10’000 EUR limit overrun by a couple of euros - will you refuse a sale in such case?