Boiler Replacement in NJ

I need help! I have a very old and very drafty house with an old oil-fired boiler that feeds a 1-pipe steam system on the first floor and hot water baseboard on the 2nd. The boiler also provides the domestic hot water. We are going to replace the boiler with an gas-fired unit. We would love to go with a direct vent system (our chimney is also in bad shape). We are considering converting to all hot water (but keep using radiators on the 1st floor) or sticking with our current set-up. I have bought the books from this site and talked with a number of "pros" that didn't leave me feeling very confident. Do I need an engineer or should I keep talking to plumbing/heating guys? I am in Northern NJ.