Hi Eddie
I am just finishing off a tich and i have struggled like mad on it (too small as i didnt make it slip eccentric!!)
I am the same as you as i am not a machinist i am a carpenter, I have tha advantage of my dad who has built 5″ locos and is a very skilled engineer
If i was starting again i would look at something like a sweet voilet (3.5″)or sweet pea(5″)
mainly using blackgates
tich is ok as an engine but i have spent 4 years on it and at my club the track is fairly big and me being on the festively plump side and my daughter whats a ride on it !!
for a bit more work you might get a more usefull engine
sweet violet plans from blackgates are £20 and oinly 4 sheets
simple as hackworth valve gear and a marine boiler
but above all build something YOU want to build
(i have no conections with blackgates)
hope this helps
Nigel