I agree. Ranchu are adorable!^-^ Now that I've gotten the squee out of my system...
I've never seen a 40 breeder in person. What are the dimensions? How is it compared to a 55 gal?
For substrate, sand is a good choice. Either you want something that is too large for a goldy's mouth or something that can pass through their system unharmed. Sand is a good choice.
Here is what I would do about stocking: get two first. We know this tank can hold two. Check your parameters (nitrate, pH, ect), and see about what size water changes you need to do to keep the nitrates down under 20 ppm. If it doesn't look like you need really large changes (over 45%) then you could try a 3rd. Those little guys don't need as much swimming room as other varieties. But be prepared, if your water quality looks like it is decreasing too much, you'll have to get rid of one. Overstocking a tank is the death of your fish. I would actually keep out the mystery snail as they have large bioloads, and with goldfish you don't want anything else adding to the bioload because goldfish are already little poop machines.
One other tip that I don't see very often but might help your ranchus is to baffle the filter flow. With two 45-gal-rated filters running, you're going to have a bit of flow. Since ranchu aren't the greatest swimmers it would be a great idea to lessen the flow with sponges and water-bottle baffles. I did it with my guys (veiltail and ryukin) and is has made a large difference; they no longer fight the flow.
I wanna say I have seen ranchu without wens, but it's not a common thing. Maybe they were called lionheads or something(?). I also think I've seen oranda with long, flowing fins, but they always have a wen (it's a breed standard). After the experiences I'm having right now with long, flowing fins on my veiltail (finrot like mad), I'm not inclined to seek long-finned anything out. Remember, oranda get much larger than ranchus and you might only be able to keep one in the 40 breeder. In fact, the largest goldfish on record was an oranda.