Hi Chris,
Welcome to A.Org
1. Yes, A1200's have a header for a PATA 2.5" laptop drive, but any size above 4Gb means patches and so are harder to install. You would need the cable and the platform to fit it normally.
2. Yes, RAM card OR accelerator with RAM. 68030 with RAM works with most programs.
3. I believe OS3.9 will go on it, but I don't think there is much point unless you have a graphics card. I have OS3.5 on mine which works well.
