Depends on how you want the zed to look. Ideally I'd want big bodykit, chunky wheels and a spoiler. If your thinking subtle I'd say spacers for starters, make a huge difference to the zeds presence, maybe a lip for the bumper, rear spats and subtle skirts like the type buster has?
Some people have an overall look they want to achieve, some people buy mods individually on impulse as they go along