Re: Original instruments
Furono used to be leaders in radar and sounders and are still very good butmore commercially focused these days and for recreational use others have caught up. Navico (B&G, Lowrance and Simrad) vs Raymarine is really the main choice and it's down to personal preference.
Pluses for Navico are the 4G radar and the autopilots are good. (I believe Navico took over Robertson quite a while ago so that's their heritage.) B&G radar and autopilot computer are identical to Simrad.
I don't think Navico do a rotary autopilot drive though so Raymarine might win there, although a Navico computer and control head should easily operate a Raymarine drive. Raymarine also has an arrangement with Apple making it easy to use wifi to use an ipad as a plotter repeater if you have a Raymarine plotter, although I think Navico can do that with an extra item.
You should be able to convert to an N2K backbone to integrate B&G (or Simrad) with Raymarine but linking Simnet and SeatalkNG needs adaptors and thought. Although some information appears to be sharable between brands it may not all be, so it 's a bit of a minefield, such as getting the autopilot of one company to steer to the wind instrument of another company, or steer a route from another brand's plotter.
If I was in your position I would probably go with all B&G as your mechanic suggests, unless the autopilot drive was a determining factor, but it is like choosing a brand of car you like, each has their fans,
John, Maramu 91, Popeye (Furono radar, Raymarine St60 instuments, Simrad autopilot, Lowrance plotter)