I have no problem with non-breathable (silnylon) rain pants. If there is sweating, it will be around the waist and hips, and you might want to provide some ventilation there.

The only problems I have is when it's warm and wet and I'm actively hiking. If it's warm, I leave off my rain jacket and pants and just get wet, since my body heat will dry my nylon hiking pants and shirt (or my baselayer top) in 15-20 minutes. When it's warm, even breathable rain gear produces a sauna effect that leaves me wetter than the rain. YMMV, of course.

Definitely use a flat fell seam for the crotch even if you don't for the sides. As you probably know, cheap trousers inevitably give way in that area first!