Mechs, I would say, are actually the among the more difficult options these days. They could be said to be inferior, outdated devices. I'm of the opinion that they're not bad choices for new builders, as you will be forced to learn about the limitations of the device and build to it, which will improve your intuition when it comes to setting the parameters of your builds. Basically, if you can learn to build effectively for an RDA on a mech, then everything else will be a breeze. You will know exactly what you're doing and why you're doing it by the time you graduate from one. It's sort of what you might call the archetypal dripping experience. It'll give you a better idea of what it's all about.
It's not really that not many regulated devices can sub-ohm. In fact, a large portion of the boxes on the market can easily sub-ohm and some can safely go lower than would be possible on a mech. The point is that it's not necessary to sub-ohm in order to get a sub-ohm-esque type of vape with a high-powered box..
High-wattage isn't all about clouds. RDA's just tend to perform better on all fronts at higher power levels than other devices because the chamber has much more space and airflow than say, an RTA or a clearo tank, which tend to perform better at lower wattages because the coils are packed into a tighter space with less airflow and sacrifice some efficiency for juice capacity. That's not to say that you can't build to 20w on an RDA and do something decent, but you'll be limited to small single coils, which will severely diminish your ability to experience all that dripping has to offer in terms of flavor and texture.
There are reasons why people say that once you drip, you never go back!
High-powered boxes can be gotten for well under $100. I'd recommend looking into one of those. Even if you don't think you'll need the headroom, it'll be there just in case you feel like experimenting and battery life will likely be better than low-power, single battery devices at the 20w range. Something that you can grow into as your needs change is worth the investment. Not having to jump from device to device as time goes on will save you a ton of money.