One question to ask is … Are you happy about building totally from a drawing? or do you need some construction notes, manual etc?
Also take a look at Blackgates, they do the Drawing’s & etchings/castings.
If the latter then you should choose a loco that has a set of construction notes published in either book form or serialized in one of the magazines.
Also do you prefer Standard, Narrow Gauge, UK or Oversea’s
Also do you have any railway interests? It is better to build a loco that you like or have a soft spot for.
A lot of questions … but you need to make the right choice, or you will just not finish it.
When I made my choice, I did not want to work totally from drawings, and I like Dougal from the Welshpool & Llanfair loco’s (Reeves Drawings), Darjeeling ‘B’ Class (In ME) and Quarry Hunslet’s ….. I choose the Darjeeling ‘B’ Class … (I also picked up a Hunslette chassis – No cylinders cheaply so will be building that in parallel)
Dave