What type of wood filler should I use?
Wood fillers for filling grain are very different from those used for structural repairs.  Grain fillers are diluted and rubbed into the wood grain.  See the attached listing from Constantines, one of the leading woodworking suppliers in the U.S.:

http://www.constantines.com/pastewoodfiller.aspx

They are also a source of components and books on building guitars.

I do not know of a non-U.S. source, but they may ship internationally and their info may help evaluate local resources you find.