A thorough installer will walk the whole house, measure rooms, look at windows and insulation, and check the electrical panel for capacity. In older Scituate homes, they'll often look for chases, closets, or attic space to hide line sets and decide between ducted air handlers and ductless heads room by room.
Outdoor unit placement is where the coast comes in. Expect a proposal that keeps the unit above likely flood water, out of drifting snow, and away from where roof runoff lands. Salt air is hard on coils and fasteners, so ask whether coastal-rated coatings or stainless hardware are available.
Most installs need an electrical permit and sometimes a building permit from Scituate's Building/Inspections Department. The town accepts those applications online only, and the contractor typically requests the inspection through the same portal.