As someone that uses it 2 computer set up iMac 2009 and iMac 2007 I would take a look at the following.
Teleport
http://abyssoft.com/software/teleport/
Allows you to use one keyboard and mouse or trackpad or all three. It also can allow you to synchronise the clipboard and drag files from one computer to the other like you mentioned above.
Not recommended for large files, I would use AirDrop if available or configure your sharing settings also good if you've got external hard drive's which I would recommend plugging all into only one machine unless there for Time Machine backups for that machine only or another type of backup.
If you're connected to Wi-Fi to get Internet access, I would recommend joining the two machines together with an ethernet cable this will require some set up in the network system preferences pain. Also see below for the security portion.
Can't exactly remember the audio setup currently will have to have a look later.
I would recommend you set up a self signed certificate from with in Teleport as you don't want that sort of information that's getting transfer between machines sent in the clear.
Also the machine that doesn't have the keyboard and mouse set up you need to use automatic login as the Teleport application doesn't run at the login screen.
I have the computer set up to lock the screen and require a pass code which is triggered by my login items by an application that you can find for the current OS your running most likely called "SleepDisplay" if you don't want to use this method setup screen sharing and log into this machine manually each time.
