So what I've seen for Ivy's is this: either bodypaint or tights works. A bodysuit is probably not going to give you the look that you want (naturally green skin). If I were you, though, I'd look through some poison ivy cosplays and see if you can find someone who used a zentai or zentai-like suit so that you can decide for yourself if it'll get you the look you want. I, personally, would reccomend against it.
Really if you want green skin then body paint is, unfortunetly, the way to go. PAX is excellent, though difficult to get off. Maybe a mix of tights and a matching bodypaint? Then you'd only have to paint the exposed skin.