I'm not sure why you'd want to re-register a boat under US colours if you aim to buy it in the EU and presumably wish to sell it 5 years on? Leave it under it's existing flag for the duration...............
FWIW most of the boats one sees in the Med under US flags
are not owned by US people, but rather bought by a local (Greek / Turk / Italian) from a previous overseas owner, and then registered by that local usually via a company in Delaware or other low taxing US State and thereby avoiding any VAT payment at home.
Re VAT, I can remember getting advice that even a EU VAT paid boat that leaves the EU for more than (think it was 4 years but happy to have someone else clarify that) could be liable to pay it again on it's return. Reality is I've never once heard of this being applied but that professional advice was that it could be.
Good luck in buying