Was a smoker for 16 years. Pack and a half a day of Marlboro Lights. I was able to quit using an ego style set up back in November. To be specific, it was a generic, 650 mAh, ego style, variable volt (can adjust power by clicking the fire button 5 times), passthrough (can vape while it's charging) battery that came in a blister pack with a generic bottom coil clearo (tank where the heating coil is on the bottom). For a moderate to heavy smoker, I would not recommend anything smaller then an ego. Battery power and vapor production would make a pretty unsatisfied smoker with something the size of a cigarette, so go for something the size of a cigar (ego) at least.
I also recommend variable volts, as I was able to increase the power when stronger urges hit. This proved to be pretty valuable to me. Now that I'm pretty much out of danger, I still enjoy variable volts because different flavors taste better on different settings. Also, a coils resistance (ohms) can differ, even if you get coils that are supposed to be the same as what you are used to. Generally, the higher the ohms, the more power you will need. A variable volt device will let you adjust accordingly.
Finally, I recommend a glass tank or clearo to all new vapers. One of the easiest to maintain, clean, etc is the Mini Protank II by Kanger. I also recommend the Aspire BDC ET-S. The Aspire will be cheaper, but the replacement coils are typically more then single Kanger coils. Some say they produce more vapor and flavor however, so it may be worth the money. Also, I believe people have recently had less issues with aspire coils then Kanger coils.