Things like this never have a single
answer for everybody. There are lots of considerations.
Does the inverter supplied by Lagoon meet your needs? If not, install your own.
Will Lagoon warrantee the inverter and its installation
? Does that matter to you? One stop shop repairs
for a year (or whatever...) can be handy for a new boat
If you will pay someone else to install it, Lagoon might actually be cheaper since they have standardized parts
and know how it all goes together without thinking...
If you have the tools (for example, heavy duty lug crimpers) then you are halfway there. If not, there are extra costs to DIY beyond materials.
Is Lagoon custom electrical
work up to the standard you want?