If you use it for music, movies and browsing, you don't need a keyboard. This will surprise you if you havent used a tablet before, but is seriously the case. Even basic emails are easily done with touchbased keyboards.
If you already have a monitor, what is it connected to? If it is an apple, then iPad streaming over wifi should be simple - though admittedly I havent done it myself.
It all comes down to what you want to do with it.
If you seriously only browse and watch, get a tablet. If you want a big screen, either get a big tablet or some way to project it (I agree, the cable doesnt cut it, but there are several alternatives). I've also seen friends use their monitor on their desk during the day, then simply swivel it in the direction of the couch for the evening. If you want to use Word/Excel or email for business purposes, get a laptop.