Hi Frank,
I agree, building a standalone multipurpose OACSaccounting package is like inventing the wheel. The export function however is the way too go. Having siad this we still have the problem that daily work done in either an intranet or logger/projectmanger will have to end up in the accounts dept. being based in the Netherlands, Twinfield seems to be a solution accepted by major accountants. In that respect it might be interesing to look at Twinfield, hence my reason to go for this service. From what I have read sofar they seem to cover the accounting systems outside Europe aswell, but that's something I need to dig in some more.
I hope to have Twinfield up and running next month, and will create a demo acocunt for testing purposes. From what I have read this is more or less in line with your German and Spanish systems. I look forward to work with you in finding a workable solution.