Should I make a mail coach?

The best thing to do would be to look on eBay for a Hornby model, being a british company they produce the most accurate versions of british outline railway models. They also produce them in OO gauge which is slightly larger than HO as produced by Bachmann and the european companies so it would be more suitable for modeling a Tomy equivalent. Bearing in mind your need though all you need to do is look for a non-runner and possibly an older less detailed version also in which case you could probably pick one up fairly cheaply.

Out of interest what chassis are you thinking of using or are you going to manufacture one of those also?

I'm unsure on whether or not I'll recycle a chassis, or 3D print every single part I need including the chassis parts. It'll probably be an ordinary Thomas drive unit, or most likely teh drive unit of an engine like Henry or James.