To answer your question about the photo and why I have so many trimmers and blowers etc, as I said before, buying or adding another trimmer or edger or blower to a business isn't going to necessarily make you or him any faster, it has nothing to do with our speed per lawn. The only thing you accomplish is having back up equipment, which IMO is a necessity. It's called being prepared to do any job the client needs to have done. I carry sixteen pieces of equipment on my trailer everyday when I leave the house.
As far as the mower goes, with you having only 12 lawns or clients to do in a week I'd stay with what I have and maybe take a look at getting better equipment like Blowers and trimmers and such because 12 lawns doesn't justify spending 8K on a stander or 2 walk behinds. IMO you need to stay away from the commercial end and stick with the smaller residential stuff and build your client base to maybe 25 or 30 then start looking at the Stander or walk behind. As far as mowing hills the Grandstand is rated at or for up to 20 degree slopes, a lot better than the 15 of the ZTR.