I mainly used Toddy and other full immersion techniques.
In the place I work we just use the Toddy. For big batches it is in my opinion the best alternative. We have a drip stand (Hario or some other fancy wooden thing) as well, but it's just not as good, because the water is dripping on the same spot of coffee grounds constantly. You can mitigate that by placing a round paper filter on the grounds (from Aeropress e.g.), however there will still be some uneven extraction. An alternative is the Filtron system, which is basically the same thing. What you could do as well is just brew it in big jars or barrels and then filter it in 3 levels with a restrainer, cheese cloth and coffee filters.
Here is a nice little comparison by Stumptown coffee, that places the Filtron and Toddy system on the top.
To be honest, cold brew works best with washed Ethiopian/Kenian light roasted coffees in my opinion, which really brings out a lot of sweetness and a nice floral flavor. Cold brew generally reduces acidity, so it works well with high acidity (african) coffees. Sumatran coffees, which would match your flavor requests, are already very low in acidity so the cold brew method can't really shine here. You can also try natural processed african coffees, they would be more full bodied, chocolaty and with notes of red berries (had an amazing ethiopian natural that tasted incredibly like strawberries). They are not really my thing though, a bit flavor overkill most of the times.
Central American coffees usually work quite well as well, with a bit more nutty, caramel and chocolaty flavors.
I've written something on this in this question already. In any case I use very high coffee to water ratios and don't dilute the concentrate (just served on a lot if ice). However for many people that is too strong and they dilute it with water or even add a little milk, which works quite well with this kind of concentrate.
Depends on your taste and the best recommendation anyone can give you here is to try a lot of different coffees and recipes until you find your style. You should definitely do some testing before you sell, so you actually like what you are selling and understand what it is. Generally longer contact time means higher extraction, 12 to 14 hours is what I do at home (in a mason jar) and I like it most. In the shop we do faster brews though.
Well I hope that helped, I'm happy about comments and suggestions how to improve the post.