Hey Mattijs,
I don't know what local shop you're talking about, but since you're from keerbergen, there are 2 shops with a rather large stock not so far away from you.
The first is Albatros in Muizen (200 meters before Planckendael) en the other one is in Grimbergen (DoehetzelfTAM)
I guess both are within 20-25 km of keerbergen.
Check these sites for adress/phone/opening hours
http://www.albatros.ws
http://www.doe-het-zelf-tam.be
My guess is a 8x5 prop (3-bladed) will be better on the .25FX
Greetz,
Dennis