S.B.
My friend did that in her playroom and also in her garage. I'm pretty sure she just drew it freehand on the wall and then painted it in. But it all depends on how you are most comfortable doing it. If you are confident you could do it freehand and get all the proportions right, then go for it. Otherwise, it would probably be smart to do the projector at least until you get a feel for it.
Do you have any friends/family that would like to have a nursery mural? If so, I would start there. Most likely they will have other friends over that will see your work and ask who did it, then they can give you a referral. You could also advertise on craigslist or in the local paper. If there is a boutique baby store in your town you could see if they would let you post an advertisement near the register.
I really don't know how you should price the murals. I would think it would depend on the size, the amount of detail, and the time it takes to complete it.
Good luck, this sounds like a fun venture for you!