RE: Is there no problem being all the zipcodes in the Netherlands?
There should be no issue, but, the addresses must be geocoded as it is the lat/long coordinates which are used to determine if an entry is within a radius. Connections attempts to do this automagically when an addresses is added to an entry.
To do this manually, edit an entry, scroll to the addresses and click the Geocode button IF the address lat and long fields are blank. That will geocode the address and at that point you can drag and drop the map pin to tweak the address location (if needed). If the address does have the lat/long fields populate, you can still click the geocode button to tweak the coordinates.
Also, I should mention the default radius is in miles, the unit can be changed to KM.
RE: is it possible to just filter on zipcode/radius, not showing the other input-fields like categories, state, country and keyword?
I have not tested that specifically, but it should work just fine. The other fields can be turned off.
Hope this helps!