Cheaper manual wideangle alternative to Pana 20mm

I've read several place that a good rule of thumb for using old lenses designed for film cameras is that with focal lengths below 35mm you start to get a lot of distortion on M43 (and APS-C) sensors. It has something to do with the angle at which the light rays hit the photosites on the sensor, particularly those near the edges. Lenses made specifically for digital supposedly adjust for this by virtue of their design.