Hi, npc's will interdict your ship if you are carrying anything or have accepted a mission where it says there may be hostile ships in pursuit. Indeed, they can interdict you at random regardless. Just remember that for you to attack a hostile npc and not be attacked by the law, the npc must be flagged as WANTED, which you can discover by targeting them (screen bottom-left).
From experience I would say yes, that your rank, ship and cargo will indeed affect the type of foes you encounter and also the reward you receive should you triumph.
Also, in the chaos of battle, accidentally shooting a police ship will instantly flag you as wanted and you will need to leave the area to survive.
Best place to start and test out combat is at your local Nav beacon (found close to Sun and listed in the Navigation window). Following this, you can try resource extraction zones and combat zones when you become more confident (read: richer).
Want to earn additional bounties by shooting WANTED npc's, then outfit a Kill Warrant Scanner, and scan a wanted ship prior to destroying it.
Playing in Solo can certainly be safer (and less laggy) than playing in Open but nothing beats the thrill of seeing and interactiving with others and even being part of a Wing!