sounds like you are looking for something in the 30 foot range (longer than a shorty, shorter than a full size).
You are probably looking at somewhere in the 10 mpg range, although I have heard numbers as high as 15, but that seems rare. You might do better than 15 with a van chassis based shorty.
I am still a wanna be skoolie owner, so I can only pass on what I have read here. I do have a 21 ft toyota motorhome and I can tell you that I can drive it almost any place a car will go. It's main limitation is height clearance. So, no drive through service at McDonalds. I think if you stay under 30 feet, your access to most places won't be that limited.
As for toilets, they are possible, but, certainly add to the amount of work you'll need to do. If this thing is gonna just be a hangout for your buddies or something to drive to the beach, you might want to skip a toilet or go with a portable porta potty type.
As for interiors, wood paneling seem kinda popular. I would think a 15 year old might get his ass kicked for even talking about hanging wallpaper!!!
Paneling is cheap, light and doesn't need anything over it for a finished look. I wouldn't put it in my den, but, it'll do fine for a skoolie.
Search through the various forums and photo gallery. You will get lots of good ideas/info there.