Adding to the conversation about buying second hand kit built locomotives here is a list of what I have bought on eBay recently.
Gem fowler 2-6-4t which includes wheels gears and motor for £60.
K's coal tank which again has full working chassis but missing its chimney for £20.
Gem 56xx on a triang chassis (I intend on this being my Guinea pig for building a comet models chassis) £15.
Gem LNWR g2 body kit £21.
Craftsman midland 1f which is a failed build and got a lot of glue on the chassis, also requires a motor but has a full set of wheels, £40.
Quite happy with that lot, just need to build them when I find the time.