Generally for Portraiture Photography, which this sort of would be like, lenses between 80 - 135 is considered to be a good length. But as John said 50mm would be fine, as long as it isn't a prime lens, otherwise you haven't got the freedom to move around as much and zooming would be better as well, less distortion as you are worried about.

I take ti the lens you are using is structured for Digital Use, like some of the new small lenses for the Canon Digital cameras, so the true mm remains correct.