1) Should I go with dual 4.3 L (205 HP) mercruiser or a single 7,4 L
(454 HP) and what's the pros and cons of having 2 engines instead of
one ? I can't make my mind about the fuel overall cost.

For what it's worth, I've heard from owners their dual prop (e.g. bravo
3, volvo) gives improved efficiency/performance/tracking over a single
prop ... or ... almost as good as having twin engines. So I'd test
drive a dual prop. Plus there's a lot less to maintain/go wrong ...
about 1/2 as much :-)