I haven't actually used Powerline networking myself, I have to admit. But that would be my first choice rather than wireless if I needed it. I have helped out several friends with wireless and seen lots of problems there (very poor range, interference, unreliable routers etc).
Yes, you can pretty much just plug more in if you want to add more devices to the network. I don't know what the maximum is, but I don't think it's likely you'll hit it.
I don't know too much about Solwise, but they seem to be doing pretty well in the Powerline networking market. If you feel happier about going for another brand, I'm sure they'd be fine too.