Hi Henry,
I am in Australia and nearing the completion of building a traction engine. Mine is a 6" Ruston Proctor SD, a bit smaller than a 4" Allchin. Although the material costs are less for a smaller engine, sometimes the manufacturing time can take as long as a big engine and making smaller parts can be very fiddly.
The prices that Jason has listed for the Allchin is a good indication of what it will cost to buy components. The main cost is the labour involved in building an engine. I have been building mine for 5 years. Material costs are less than $5000. BUT if you add labour for 1000+ hours work at $20/hr (and where do you find a machinist at this rate), you end up with a total value of $25,000+.
If you intend to build one, the first thing you should do is to join a model engine group and see what size engine would be best for you. Then get a mill and a lathe. Not forgetting tooling.
Used model engines sometimes come on the market but are rare in Aust. There is a 1" Mini on sale on ebay at the moment for $5000 but a bit small if you want to tow youself and a mate.
https://www.ebay.com.au/itm/Steam-Traction-Engine-1-12-scale-Minnie/273166855859?hash=item3f9a036eb3:g:7sEAAOSwB4BaCaUa
Paul.
Edited By Paul Lousick on 09/05/2018 09:08:55