there was an article in MBY about 4 months ago about this very subject.
from memory ended up about 2/3rds of costs when you include VAT and import duty, shipping in UK and US.
there were some questions raised about CE conformity and the like as US supplied engines (regardless of manufacturer) are (deliberately????) marked without CE to avoid such shennanigans.
as an aside I imported a boat from the US last year with few hitches (Sunseeker though so no question of CE marks etc)...if you find the right deal then do it!!
I have a local man who brings mercs in regularly. I'll send you his number via PM. But you can have warranty issues. Merc in UK is Barrus and I don't think they will support "grey" imports.
The hole in your transom will be too big for a Merc drive. You'll need an adapter plate or some serious remedial GRP work carried out......if I reading it all correctly that is!
Would look that way although the plate is a easy and neat option. Will add to the cost though
Another way of doing it is to use a volvo bell housing off of a V6 and a V6 merc keeping your drive, if its upto the job. You will have to sort the raw water pump though as the mercs have the raw pump in the drive. No pump in the volvo drive, but you knew that already.
If it were me I'd go the vilevo route as it'll be easier re your transom. Theres a few dealers out there who do recon warranted engine/drive packages. Whatever way you go prepare the wallet for a bashing and good luck.