Can't be 100% sure exactly what rules you'll have to comply with in
France, it may not be exactly the same as in the UK
Are you paying VAT on the boat itself? If not then it should be possible to avoid paying VAT on any equipment shipped to France for fitting to the boat. This is only going to be the case, however, if the boat is for immediate export to a country outside the EU (which I presume is the case)
You will, I would anticipate, need to get the VAT and
import duty exemptions sorted out in advance. Without the relevant paperwork the assumption at the receiving port (air or sea) is likely to be that duty or VAT or both is payable on
import. If you're talking about thousands of, er, dollars (whatever they are

) worth of equipment it's going to be worth getting expert assistance as you suggest
I can't offer any suggestions as to who to use I'm afraid, no experience of importing stuff into France. Might be worth asking the boatyard /
boatbuilder as they may be able to recommend someone. If they are selling regularly to overseas customers it's unlikely that you're the first to want to have your own
gear installed
Hope that's at least a bit helpful!