Setting Postfix up as a smarthost's pretty straightforward. I've done it in reverse as an inbound mail gateway instead of using an MS Exchange Edge Transport role. I'm guessing you'd be allowing access based on IP?
While you want to keep it simple, I can't stress strongly enough to use at least outbound spam filtering. That way, in case you do somehow start churning out spam, it'll get caught before your IP gets blacklisted.