Its pretty easy to swap out old for new.
Maybe try a smaller car audio shop that may be willing to do the install work for a lot cheaper.
But, if you are going to relocate the fuse panel, you are going to need to run some new wiring, which would be better than splicing in extensions to each wire.
Also, make sure you tell them if you want them to use marine grade tinned wire.
Also, decide if you want them to use waterproof connectors, and shrink-wrap all terminal ends.
To do a real professional marine wiring project can get pretty involved and pricey.
If you use your boat occasionally, to snag a bass or crappie, and keep the boat garaged when not in use, you could probably save yourself 50% or more.