Some depreciate worse. Depends on manufacturer, location, floorplan, season, market, etc.
Some things can be easy. Those ext. speakers; hard. Wiring is run in the walls and ceiling, you DON'T want to do that. And don't just staple it to the wall. Some things are easy, like an electric tongue jack. But factory options are cheaper from the factory.
I guess your warranty could technically be voided because you aren't a dealer, but when I do warranty work, I use common sense. A light added under the steps didn't break a cabinet in the bedroom for example.