Will I be able to play most old games like Giana Sisters, IK+, Bubble Bobble, Ports of Call, Defender of The Crown etc? I've heard there's some kind of boot menu that can trick the games to think they're running on an A500 and the likes of that machine. Does this work for most games? Or will I be better off buying an A500 if I want to play these games?
You can't make the game think it's running on an A500, however you can disable the CPU caches and emulate the older chipsets to increase compatability.
If I want to add a HDD in the future, will I be needing some kind of interface to attach an ordinary IDE-drive or is it just to plug it in and boot?
There is a built in 2.5" IDE interface on the A1200, however it's not very fast. It should work fine though.
I've read about problems with the floppydrive in the late/relaunched A1200s. Any experience with these?
Yup, there are quite a few old games, who won't run on these machines. Games from 1989 and up generally works though.