The M403 is a good model, but there's a catch.
It's a super-mini, which means it's literally cigarette-sized. Due to the limits of technology, that means it doesn't have a lot of power.
The batteries die quick. The carts run out of juice quick. And it just doesn't deliver as much vapor as slightly larger models.
Is it a good e-cig? Yes, for a super-mini. Is it enough for a heavy smoker to completely give up cigarettes? Probably not. At least, it wasn't for me, and it wasn't for most of the people here.
I wouldn't recommend you get a super-mini for your main e-cigarette, as a heavy smoker.
For a solid starter model, I would recommend either the Kr808D-1, or the Joye 510. Both are "mini"-sized, which means they are slightly bigger than a 100 cigarette, but still cigarette-shaped and small.
The Kr808D-1 is a 2-piece system with a battery and a cartomizer, which means it's very hassle-free. It has the best battery life of the mini e-cigs - I get 4-6 hours out of mine. The cartomizers last me about 3 hours. It produces strong throat hit, and has a warm, ritch flavor to me. However, it's not the best vapor producer around - it's adequate, but not spectacular.
The Joye 510 usually comes as a 3-piece system with a battery, atomizer, and cart. It produces plumes of vapor, cooler vapor with a "cleaner" taste, and a softer throat hit. The battery life is not as good as the Kr808, however, and the carts need to be refilled somewhat frequently. However, they do make cartomizers for the 510, which I quite like.
Either of those kits won't cost you more than $50. With that, you should get a spare battery, spare atomizers/carts or cartomizers, and juice. When you get juice, get a couple small bottles in different nicotine strengths, to find your "sweet spot." I'd recommend you try 18mg, and 24mg.
I got my kit from ElectroNicStix.com and had a very good experience. You can get both the Kr808D-1 and the Joye 510 from them, and their prices are reasonable. They also sell Boss juice, which I have heard good things about.