If you want to play PS2-games maybe it would be easier and cheaper to just buy a PS2. And PS2 is also backwards compatible with PS1 i think.
This isn't a bad idea. You could get a PS2 slim, which will play PS2 and PS1 games. You already have a X360, which is essentially a better version of the PS3, and a PS4, which has several remastered PS3 games.
That said, Demon's Souls alone is worth ponying up for a PS3