Hi, Sketty!
There are several sites and videos on the web to show installs of in wall speakers (e.g. Crutchfield has some information
here, and there are videos on Audioholics and YouTube). Checking those out can give you an idea of the work that's involved and if you're comfortable doing it (or hiring someone to do it).
In wall speakers are a bit more difficult than on wall speakers because you need to cut into the drywall, and you'd definitely want to run the wires inside the wall (which would be more optional for on wall speakers). The wire routing through the walls would likely be the most difficult part, but people do it all the time. A couple of years ago, I installed wall plates for my speaker wires and was hesitant at first because it involved cutting into the drywall, but it turned out to be quite easy. I didn't have to route the wires very far, though.
One thing to check is if you could locate the speakers where you'd want them. I'd be comfortable placing them between studs, but I'm not sure if it's wise to cut a stud to place a speaker (I should check out those videos, myself
).
Others here are much more knowledgeable about this, so I'm just getting the ball rolling.