You could always try a kit. This supplier appears to be incredibly cheap for recognized brands.
Das Elektrorad von Frank Scheftlein: Komplette Pedelec- / Elektrofahrrad Umbausätze und Ersatzteile von Bafang, Puma (eZee), Tongxin u.a.
at 150 Euros + VAt all you have to pay extra is for a...