I think for architecture MoI is fine and a perfect choice.

If your interested in modelling vehicles you'll need a good polymodeller due to the complex compound shapes involved, MoI really doesn't have the toolset (currently) to make this a easy task. If you interested in simplifying the shape of vehicles MoI would be fine for that.

I doubt you'll find a easy rendering engine as this is a complex area. Vray is certainly one of the best and almost the standard used by architectural visualisers. I found it pretty easy to use but I'm quite familiar with Mental Ray and its materials so switching to vray is in many ways similiar.