Merican NWG etched coach kits should do the job.
The kit looks simple enough - let's face it, a coach is only a box after all. One snag is that the packet of whitmetal bits for one coach but these are in the post. There isn't much in the packet mind you - the bogies and some 6BA bolts basically.
The etched paneling on the kit certainly looks nice. My brief is to build the models and paint them "too look old". The box (which is big enough for the finished model) shows a plain brown livery that I ought to be able to replicate.