Hallo Gertjan,
I dont't know how the manuals are wired... You can try http://www.heyligers-orgels.nl/ (Musitronics).
If you remove all electronics you can either make a scanmatrix or scanrow! (Depending on the midi controller boards you planned to use). If I were you I'll take the Heyligers!
The wooden manuals are probably made by Laukhuff (which means that there a equipped with a very good key contact system). Also the quality of the pedalboard is high. The manuals are easy to wire (I have some Laukhuff manuals here).
I think the Johannes has Fatar manuals? I personally don't like the "plastic" touch manuals used by Johannes.
There is also an other option (replace the manuals of the Johannes for Laukhuff manuals):
I know someone in Veenendaal (NL) who sells used Laukhuff manuals for fair prices (around €50 /manual).
I purchased mine there too. I can send you the contact details.
I don't like the Heyligers consoles by the way, they look old-fashioned and very simple