What sized bottle are you getting filled for 12 bucks!?!?! 20# is like $30 here now.
And yes, soapy or starsan water is your friend. Minimize connections in the design. Dont skimp on the manifolds. You dont have to loose that much CO2 with a cobbled together manifold before you are at the cost of the nice micromatic manifold. I have been here done this several times with several systems.
It may not be a Micromatic one ( my leaking secondary regulator was though) but I have had good like with this one from Rite Brewing https://www.ritebrew.com/product-p/843667.htm