There are many things that can affect what dildo would work best. First, how do you plan to use the end that isn't in a harness - anally, vaginally, or other? How big do you or your partner want the dildo to be? (for harness use it's best to get a dildo over 5" long unless you're using it anally, in that case 5-1/2"+ is better).
I'm with Playful Girl 100% on the material - get silicone. Wait and save up if you have to, but 100% silicone is the way to go. It's easy to clean and doesn't contain anything that messes with your body.