ScreenRecycler

Welcome to the Apple Support Communities

That application requires a VNC client, but it includes one, so you don't have to worry about.

I think the main problem isn't if it's safe (everything that uses a router or Internet can be unsafe, but if you have correct security systems, it's safe), because this application can be slow sharing screen because it uses your network. Always you can, connect your computers with a cable to share screens

You are correct. Only 27-inch iMacs and iMacs with Thunderbolt can be used as a monitor, so ScreenRecycler is the only possibility to use the iMac as a monitor with your MacBook. In your case, use it, and if you find speed problems, you can connect both computer by Ethernet

Note that ScreenRecycler uses your router, but not the Internet connection, so you won't get 0.5 Mbps between the iMac and the MacBook

You are welcome