Even though it is an existing structure, there is almost always a way to get wires in the walls or in the floors or the ceilings or the attic or basement...whatever. As deb stated, I pretend that wireless dosn't exist. When we do installs we offer wireless as a secondary convenience only, not as a solution to networking. If the client can afford it, we always push hardwire. If you don't have a drywall repairman you can contact an "emergency" drywall repairman to fix your holes. If you are concerned with getting referrals from that client than put in that extra time required to make it work. You might have to loose money on this job to make it work, it happens to everyone sometimes...it's a learning experience. Don't give up!!