@Proplus re: sweetness, look at the PG/VG ratio of your liquid. Propylene glycol is neutral in taste but carries flavourings well and is close to water in consistency. Vegetable glycerin is more viscous and has natural sweetness.
High VG liquids are more commonly used with dual- or multicoil ("sub-ohm") tanks and high-powered mods to create bigger, thicker clouds. People generally use lower nic liquid (0-8mg/ml depending on the person) but often end up using much higher volumes of it. In general the body (brain) knows when it's had its fix. If the nic strength isn't enough just add a bit of higher nic liquid until it works for you. It doesn't make sense trying to reduce vaped nicotine while still resorting to smokes.
Regarding flavours, what kinds are you actually interested in? Watermelon (max. 50 VG) sounds fairly neutral. You could simply add some pharma-grade PG to reduce sweetness, but that naturally dilutes the flavour and nic content as well. There are vaping forums and some online shops have feedback areas for discussing and rating flavours. You could order lots of small 10ml samples of different flavours and brands and simply see if anything hits the spot. Even if you end up discarding most of it, or perhaps giving the unused samples to someone else to try, it'll still be much cheaper than cigarettes.
Blowing large high-VG clouds in a small space with high humidity and possible suspended particles the vapour can cling onto may well cause the greasiness effect you're describing, although I've seen none of it using ~50-50 PG/VG liquids in fairly low-powered single-coil setups.
@wayland, the nicotine strength really depends on the person's (who's trying to quit smoking) needs. Using a low-powered device with a single-coil tank like the Nautilus I mentioned with 1.6 or 1.8ohm coils a heavy smoker may well need up to 24mg/ml juice initially. One 3ml tank equals 9 (3x3) 8mg cigarettes. I suggested getting a few different nic strengths, but with the lower throughput a very low nic liquid may simply not be enough, in my experience.
Most will sooner or later reduce their nic intake and any leftover higher nic liquid can be used for mixing down, e.g. using equal amounts of 0 nic and 24mg liquid gets you 12mg etc.
Most of the "current vaping hardware" seems to targetting enthusiasts going for large clouds (high-powered 100-200W devices using large amounts of high VG juice but at lower nic) and I would never recommend them to a friend just starting to vape.
But yes, at first there's some trial, error and adjusting before one finds the nic level that works. Wrt. the safety of nicotine, the body is good at self-regulating and at the levels used in vaping there not much more chance of overdosing than there is with caffeine while downing capucchinos.