Depends on how comfortable you feel playing cash, I wouldn't jump straight into HU, probably play 6max, read / watch a ton of content on both 6max and HU to get used to various spots and making the transition from 6max -> HU smoother.
How to choose opponents? Play them
Accept that everyone can be exploited and develop lines vs all opponents.
I'm sure there will be enough content here.
In terms of any advice, play smart and stay grounded. Value spots are the same, SD value is more important, and identify your opponents correctly, adjusting accordingly.
Regards,
Paul.
PS-- Also open your button range
In terms of cash or SNG, probably both, but aim to develop a solid cash game.