I think I would still go for the Martin Finney Kit http://website.lineone.net/~cbwesson/GWR%20City.htm Note that the photo shows how City of Truro should look with Indian red frames in as built condition, it has no copper cap chimney sandboxes top feed etc, I would like see CoT in green and black lined out with Great (garter crest) Western on the tender. The current livery and condition annoys me as much as Flying Scotsman with smoke in apple green livery.
The Martin Finney 4mm kit is ??96, you needs to get a tender, wheels, and motor plus inside working motion. So you are heading for ??200 before you start it plus it need painting and lining, so in reality the Bachman model maybe is good value for money.
David