From what I've seen, each seat will hold two adults, or three kids according to the manufacturer. So, if you have 11 windows, that means you have 22 seats. That's 44 adults or 66 kids. Most manufacturers would say you have a 66 pax bus. Also, I found a good way to tell the length of a bus behind the driver is to measure 2.5' per window. So, if you have 11 windows, you'll have 27.5' of living space behind the driver! Give or take a foot I'd say. I have a 10 window bus and have exactly 25 feet of living space behind the driver. 11 windows is a good size for a dog nose bus. 12 windows is about the most you'll see on a dog nose bus. On the flat front buses, I've seen 14 and even 15 window buses. So, if you're worried about size, you can't get much bigger for the type of bus you have. Also, I had no experience converting a bus. But I've taken a ton of ideas off of here. Look at all of the buses in the gallery. You'll get lots of ideas. I've stolen Lapeer20m's paint job, Vonslatt's plumbing ideas, etc. Nobody seems to mind!
Make sure you take pictures of your bus and the trip home!