Wow. Good for you. Shootin for the moon. I like it.
But my thoughts are that you may see less than 1% response but I would also expect more than a 60% close rate. I don't know of anyone using a return postage paid card these days but do know you need to give them a good reason to call or email you. Call to action.
Multiple hits on the same zip codes and neighborhoods are a must. I'd say at least three hits for each mail route you choose.
Other than that good luck. You're trying to add twice the amount of customers I have now. lol