If your goal is to experiment with decoction and end up with a big, malty pseudo-lager, then I think the steam beer yeasts will probably be your best bet: WLP810 or WY2112
Another option with those ingredients without decoction would be to shoot for a nice Altbier. Dial down the Pils and Munich to about 9lbs. each and the Chocolate to about .5lbs., and use WLP029 or WY1007. That might be nice as well.