If you're only using the a1200 for (classic) games then I suggest you first connect it to a big screen tv. You can always decide on getting an indivision and dedicated monitor later on.
If you still need to buy a monitor, make sure it has dual inputs: one for the indivision/sandoubler output, and one for ordinary output like workbench. Most modern monitors have dual input, but a lot have vga + dvi-d which is digital only and wont work with a normal vga signal (there are no converters for this!)
Last, a CF card is pretty cool, fast and cheap but to be on the safe side a (3.5") harddisk isnt a bad idea either and not that hard to connect inside the a1200. I do recommend a kickstart 3.1 a1200. As for accelerators, the faster the better ofcourse. But start with looking at second-hand examples. My personal preference: the Blizzard 1230IV/50.