Hi Alan,
The Reeves website used to include all the details mentioned in the catalogues but has now changed. I would agree that a copy of Blackgate's and Reeves catalogues are indespensible. Both own the copyright to certain designs, with Reeves owning the copyright to all the Don Young designs.
I will trot out my usual advice for a beginner at miniature loco building to avoid something with piston valves and a boiler with a tapered barrel and belpaire firebox.
You did not give any hint as to what you would like to build. As Neil suggested you really ought to be quite taken with a certain loco to keep you going on the project. It also depends on whether you just want something to build or something to run on a club track reliably and efficiently etc.
I have always had a leaning towards a loco that performs well on the club track, especially if it has a copper capped chimney and GWR on the side! Though the rather fiddly current project goes back to the first loco I had a ride behind namely Stepney of the Bluebell Railway, and I have drastically altered Martin Evans' 'Boxhill' design to this end. Others are quite content to produce a loco to a published design without such diversions, and some designs are easier than others.
Ask at a club meeting 'What miniature loco should I build?' and you will get a different opinion from each individual present! Some more informed than others!
Cheers,
Julian