We also have NG and we need to replace all our smoke alarms in the house. I saw a 4+ unit (EG, NG, Smoke, Fire) but it is a hard-wired model. I was thinking of putting one in our laundry area (hot water heater), near the kitchen (gas oven/stove), one in the hallway near the bedrooms, and one in the den. It will be about $200 for the set. Our house isn't prewired for a system (built in 1960). How hard would it be to install these? Or should we just spring for new smoke alarms and get a special CO/NG alarm for the kitchen/laundry?
ETA: We have a 1-story ranch. No basement to worry about.
If you are comfortable with wiring, installing smokes is no more difficult than anything else. You need 3-wire cable (usually 14/3) for the interconnected ones. Put the smokes on their own 15A breaker. In some areas (check your code) you are required to install AFCI (Arc-Fault Circuit Interrupter) breakers.
The most difficult part of the job is getting the cable from one place to the next.