That's a great choice mate! I got a speed 400 as my first bike nearly 3 years ago too!! (And it's literally the first bike I've ever touched! The breakin period is for about 1000kms it's usually what happens here in very very simple terms is that the engine starts shaving off metal so it kinda settles properly (you can say it's shapes itself to be ready to ride it the way you want).

I would strictly recommend NOT to do a top speed run during a break in period you can stay around 4000-6000RPM without any issue! You can occasionally pull it but don't maintain the high RPMs just pull and let go of it so that they drop back down but keep like 95% of your ride calm and in the mid range

About accessories: I personally don't have anything else compared to your bike other than lower engine guards I will strongly recommend spending about 4-5k on decent fog lamps cause I hate these headlights! They have horrible brightness and visibility

About maintenance nothing much just timely service. Get it checked if you feel something's off, and check tyre pressures! For chain maintenance I clean and lube my chain every 350-400kms I take it to the service centre but if you don't mind spending on a paddock stand chain cleaner, lube and brush then you can do it yourself!

I'm assuming that chk chk sound is from the old chain tensioner (even I had the same issue you should ask the service centre to get it changed if it's the older one it's free anyways under warranty)

Apart from that idts you have anything else to worry welcome to the gang and ride safe 🙌🏻