Just a little update on the filters (no I did not buy an FX or FX-N yet) - if one checks out the (rather new) http://rolleiflex.us webpage, it is made clear that FX-N can only mount filters with an additional adapter:
Quote from the FX-N description:
Special note: While the FX-N has a outer bayonet mount for hoods / shades, there is no internal bayonet for filters. To mount filters the bay IV filter adapter must be used.
The adapter costs another $410 but also allows to "swing" the filter between taking and viewing lens. To me it is unnecessary complication and disadvantage of the FX-N compared to FX.