Time for a bit of a flashback - many years ago, I built a model the Killin Pug. (The best web link I can find is the amazing simulator film) for Hornby Magazine. The main build appeared in September 2010 with the chassis the following month.
At the time, the resin body was a Deans Sidings product, but this is now available from Phoenix Precision Paints. It's intended to fit the Hornby 0-4-0 chassis, but with some extra wheels.
A better chassis was required and this came from Branchlines - but it was nice to cover both average modeller and high(ish)-fi options.
The finished model looks a lot like the Hornby Caledonian "Pug", but subtly different. One to run on your layout when you have visitors with sharper eyesight. I notice, searching for back-issues, that one forumite commented "Took me a while to work out the point of the Killin Pug, seemed to be just replacing the Hornby body with a almost identical resin one until I spotted the extra rear wheelset."!
The kit is one of those that any beginner looking for a bit of variety in their fleet could take on. Nowadays, there is a fine selection of 3D printed models offering the chance to drop a new body on a RTR chassis, and it's a very satisfying thing to do. For me anyway.