If I had your computers, I would leave the A1200s in their original cases and just get a 8MB RAM card or 030 accelerator and run a nice 3.1 system packed full of games and clasic applications, and install Aros on one of the fast PCs.
The A1200 gives you the best retro experience, and Aros will give you the most modern AmigaOS experience without needing to buy extra hardware. If you want to run OS4, you'd be best to get a SamFlex or wait for the X1000. You could also try MorphOS, which is a faster and more advanced Amiga operating system than any of them, and will give you the ultimate AmigaOS experience with full backward compatibility for applications.
To get started though, download Icaros Desktop Live, burn the DVD, and boot it up on one of your PCs, it's fully usable without even installing.