Purchase the commercial size of the HVR packets - each makes one gallon. Use buttermilk, definitely whole buttermilk. When it comes to the mayo you want to find the cheapest, richest heavy-duty mayo you can find. “Good” mayos have the problem of that weird vinegary whang (I guess they all want to be more like Miracle Whip?). A bland, fatty Mayo makes the best Ranch - we used to use Sam’s Club brand before they tried to upscale it and screwed it up with ‘flavor’, now we use our vendor’s own label brand.
I know it sounds counterintuitive, but the better the mayo the worse the Ranch. I can’t stress that enough.