I think you need to do more research before starting to build. look into ohms law for sure. If you are buying external batteries for your mod, look into battery safety. The iStick has an internal battery, so you should be good.
Using Ohms law, you will find that with the 20w device you will want to build at higher resistance to get the most out of your wattage range. I'm not sure what the ohm limit is on the iStick, but building lower draws more watts at a higher amperage which is harder on the battery and will also cut down your vape time.
Start with single coils. I think that Plume allows for single coil builds. When you get good at wrapping and reproducing the same coil, then jump into dual coils. If the coils aren't the same resistance they will heat up unevenly, and your vape will suffer.
Raise your inner diameter as a previous poster said. 5/64 or 7/64 works fine with 28ga. I wouldn't go higher than that. It's a good idea to start out with 1+ ohm single coils when starting out, keep to your iStick for now... Until you know this stuff really well, don't even consider a Mechanical mod - but if you want to build lower you can go with a higher wattage mod. They've got everything from 20w all the way up to 200+w.
Learn about how this stuff works before jumping into wrapping your own coils and such. Always check your builds with a multimeter or ohm reader.
When going to drippers, you want to cut your nic down by a level at least.
Know what you are doing before you do it. Build safely. Crawl before you walk, walk before you run...