Re: Installing a traditional head from scratch
Biggest challenge won't be installing it, but developing a secure base to fit it on and locating space for holding tank. Intake thru-hull, overboard vent from holding tank, holding tank pump out fitting on deck, and likely a few holes to run the hoses through. Normally the base is elevated to allow for comfort & ergonomics. You can build the base out of marine plywood and laminate it in place, when elevated you will be able to access underneath to install the nuts. Alternately you can build a temporary mold to create a fiberglass base (smooth surface means it is pretty as well as easier to keep clean). I'd finalize the location and build a mockup out of cardboard or masonite, then get to building the actual base.
Cost - depends, but don't cut corners using incorrect hose & fittings.
Some photos of your target location would help.