As most people on here would, I'd recommend going with some type of vv (variable voltage) before going mechanical (which the K100 is). I started with an Inferno as well, and about 8 months later got a Lavatube v2. I was very happy with it, and since it has 510 thread you can use almost any tank you can find on it. I'd recommend an Artemas tank for that. It's built very solid and has a bottom lock so your carto's won't slip out.
If you must get a mechanical, I have never heard much about the K100, but it looks awesome and is cheap so I kinda want to try it myself. Been vaping for about 2 years now, currently I'm using a Smoktech Magneto. It's mechanical as well, about the same price, bottom button, and has a magnetic button so you don't ever have to worry about loose springs or the button falling apart (some mods have plastic holding the button in which will wear out after a while). Also, the Magneto is telescoping so you can fit anything from an 18350-18650 battery in it.
You can use carto tanks for mechanical mods, but keep in mind that mechanical mods are usually restricted to the voltage of the battery (3.7v, so you won't be getting much more performance out of a mech mod other than more battery life). If you want to stick to carto tanks then a vv mod is the way to go. This way you can adjust voltage and get large amounts of vapor. It will also affect how long your batter lasts and how long the carto lasts (more voltage means they won't last as long). Get some 3 ohm carto's and an Artemas tank. I used about 5 to 5.5 volts. Be sure to prime it properly (I used about 35 drops).
Battery life is dependent on the capacity of the battery (mah). A 3000mah battery will last longer than a 2400mah battery. Size (18650, 18350, etc) doesn't have much to do with it, other than the fact that it is easier to find larger batteries with high capacities than it is to find small ones with high capacities.
If you REALLY must get a mechanical mod, then try an RBA (rebuild-able atomizer). They are a pain to set up at first, but once you get the hang of it they are awesome. More flavor and you can get a lot of vapor if you do it right. I'd go with an AGA-T2. It's a genesis-style rebuild-able atomizer tank. Genesis style atomizers are very popular so you'll be able to find tons of help with it, and the AGA-T2 is pretty cheap and very solid.
There's nothing wrong with getting a mechanical mod and RBA's, just don't get discouraged. It took me about a week to get mine setup right, which is why most people recommend going vv first.
