I agree with the rules, 1 month should be enough.
About Blender I will personnally use 100% of it for modeling but the internal render engine can't handle decent reflection plane technique. However powerful external renderers are usable with Blender such as Yafray, Kerkythea, Indigo, Povray, ..., ... to fit the rules.
So Blender is not an exception.
How many renders per entrant?