Congrats, I am building a KZ550 right now, lucky it was barely running when I got it. order a carb bowl gaskets, drop the bowl and carb clean the f out of it, be careful not to bend the floats. Get a new fuel filter and put it in between the petcock and carbs, take a bunch of screws put the in the tank (after taking it off) and shake it up to remove the rust, unless it's really bad, you can buy a kit for like $100 that you pour in and seal the rust. once that is done or I bought a fuel bottle and use that hanging from the bars so the gas is clean. New battery (bought a gel so you can remount later at any angle) look at the coils under the tank, if either is cracked you need a new one. Use eBay (I work for them
once it's running then go over the brake system, remove the dot fluid completly then fill and bleed. drive it around a bunch and make sure it's a runner before you waste money on anything else.