Indeed its not a cheap excersize, but I am mainly relying on kits for my stock, so, adding P4 profile wheels, and any cylinder mods can be done with relitive easy, I notice that, as well as Brass masters, High Level kits do a small selection of chassis, and loco kits, all are suitable for P4 EM or OO, BUT they cost a fortune... the only RTR locos I have at the moment, that are for this layout, is an LNER B12 with a knackered motor (its an older Hornby tender drive version, but the body is in good nick, and is crying out to be detailed... ) which was about £15, so a chassis, motor and wheels, maybe £70 or there abouts, and a several hours of fiddling...
I would like to see the pics of your plank please John...
Sean