Harness compatability

I'm not sure what sort of price range you're looking for or what dimensions, but for an all around good harness-compatible silicone dildo, I'd recommend Throb by Tantus. It's pretty cheap as far as silicone dildos go, and, in my opinion, a really great toy.

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.

Another big question to ask is how realistic do you and your partner want the dildo to be?