Hey moozer.
Basically, the kickstart rom is the very basic parts of the OS, stored in a chip on the motherboard, so that the disk can interface with the hardware. You'll need Kickstart 1.3 if you only want to play Amiga 500 games. I know that Retro Gamer did a special and had a copy of WinUAE with kickstart 1.3 on it, so you might be able to get a hold of it. The other option is to buy Amiga Forever which comes with a whole set of Kickstart roms, so you can emulate different systems.
The Kickstart Rom is still copyrighted, so it is illegal to download them. Saying that, however, there are many floating around on the net, and it's up to you whether you go down the 'legal' path or not. WinUAE will not operate without one though.
As adonay said, Flood is part of the 'Amiga In A Box' package (AIAB) and so you can download it for free. Here's a link to the download for the game itself.
FloodGood luck with it all. I hope what I posted didn't make it sound too daunting. Basically all you need is WinUAE, kickstart 1.3 or 2.0 (too many issues that might come up if you go for 3.0 or 3.1) and the image of the game. :-)