I would like to give visitors the opportunity to rate companies. I know there is the plugin that allows you to rate users. However, we want it to only be applicable for companies. This way companies will be more actively involved to improve their status. Could such a feature be implemented?

It would not be too complicated to update User Rating plugin and add option to "allow for companies only".

Okay that would be great. Could we implement a system where people leaving a review need to send a proof of doing business with the company? e.g. they can upload a invoice?

@Hugo
Well from privacy point of view that does not make sense.
Other side of coin is that admin would have to evaluate all these ratings that can be tuff.

I understand. However it is feasible for us (our sector). So you gues it would be doable

@Hugo
You know everything is doable, but we will always focus on features those will hit most of audience.