1) Oh, yes. AGA isn't technically any faster than ECS, but the 1200's memory architecture is better which does give you more screen speed, especially once you put in some Fast RAM.
2) I only know about the glut of AGA "enhanced" games that will work fine on OCS/ECS. I assume you don't have a PC or have a crummy one. Otherwise, emulation is a good idea.
3) Why not transfer your 600 hard drive to the 1200? Many laptop hard drives will install with the correct bracket and cable, although some come with proprietary connectors, like my brother-in-law's Dell laptop. A 600 HD should fit right in a 1200.
4 + 5) No comment. :-)