" What do I need to connect and go wireless?"
That depends on how your internet connection is fed into the phillips freeline. Is it a standard modem via usb?
" What is the likely cost and where is the best place to buy? "
Usually a WiFi router meets most peoples needs. because both your other machines are WiFi enabled a standard £40 to £60 router should do it.
" What are the security problems and how do I resolve this? "
You would need to password protect the router, this is done in the router setup. You would need to also set the wireless security. This will protect you against anyone trying to access your network. This is very easy to setup and consist as little as a ten hexidecimal code.
All can be done in the router stup programme.