I've sent you an email.
Basically, assuming it is US made, you will get an Export CofA which is basically an Annual with the CofA signed by an FAA DAR. In the UK, the DAR will charge well over £1000 for the signature so it would be cheaper to do it while still in the USA.
Then when it goes onto say G-reg, the local CAA inspector will look for any deviation from the Type Certificate. He will print off a list of 337s (major mods notified to the FAA and will chuck out all of them; these will have to be either ripped out, or recertified by a company with EASA design authority. Any field mods (minor mods) will have to be similarly ripped out or have the paperwork regenerated.
Then you get the 12 year engine life business; a > 12yr engine will need an overhaul.
Depending on the details, assuming say a C182, it could be anything from just a few k to being uneconomical..
It may be very wrong to automatically consider a
G-reg. You may want to look at other European registers. Some might be easier (Spain?) on mods, and a JAA license will cover the lot. (The CAA is very worried about the loss of revenue resulting from this kind of thing).
With any registry transfer, make 100.00000% sure you have all your ducks in a row and the paperwork is 100.000000% spotless and the destination-CAA's inspector is happy BEFORE you de-register the plane from the previous registry. Otherwise, you end up in legal limbo, with a plane which is as flyable as a chocolate teapot, and anybody who can get you out of the pickle has you over a barrel. You may even be unable to go back on the previous registry without major costs. If you are getting a firm to do the transfer, make sure they have done it before. If they haven't, use one which has. (been there done that 
)