Doing it right now.
Suggest you ask UK firm 'Tek-Tanks for a brochure as it shows all the options you could adopt in diagrams / parts
My suggestion would be to keep it all as simple as you can.
Overall and compared with other boat work I don't think you'll find it difficult.
If you do go with stainless then be sure they are sprayed inside with plastic. We've gone with a roto-moulded plastic tank this time.
Due to past (good) experience we chose to go again with a vertical gravity tank. Means no pumps to bother with - the toilet pump shoves the stuff uphill enough to fill the tank from the top. It empties from a deck pump out or you open the through hull
and out she goes.
The deck pump outlet is straight up from the top of the tank, and the through hull outlet is directly in line with that, meaning should one ever get a blockage you can push it out via a rod from the deck.
Re costs - we had to go for a custom made tank, plus vent, charcole filter, inspection hatch
, deck pump out fittings - lifting our cost to just under UK£600.
Had we slightly more room we could have used a standard plastic tank from Vetus or others and saved over UK£200.
I am fitting it myself.