If you are an AOPA member, they have an airplane value application on the website. Just put in the TT the engines have now, then the TT you expect to have when ready to sell it off.
Have you looked at the Beech Dutchess at all? 140kts on 16 to 18 gal/hr (If i remember correctly). The Seneca One would also be a good time builder, and you can usually find these pretty cheap (at least compared to the other Seneca models). Also, FWIW I would stay away from the more advanced Senecas (2 thru 5), there's no need for a turbo-charged time builder - just more parts to replace when the thing breaks down.
An O-360 would cost on average to overhaul about $15k, so you can calculate how much value you would lose in the airplane. WIth TBO of 2000 hrs, it would be costing you about $15 and hour (15,000 x 2 all divided by TBO). So 400 hours of flying would decrease the resale value by about $6000, for a very rough guess.