Also consider the manufacture date of the gun.
Pre 1899 guns can be shipped without the need for an FFL, which simplifies things a bit if it does not ship through a customs house here in the US. In fact, it may be possible for the auctioneer to ship a pre 1899 gun directly to you - I'm not sure. If so, this could cut the cost dramatically.
Holt's have this on their website:
http://www.holtandcompany.co.uk/US%20Import.doc