@Null
Some options here (because "good" is hard to parse when there are so many accessories)
Light:
Streamlight ProTac HL-X for a rifle, I don't put lights on my pistols so somebody else will have to chime in.
Sling:
Pretty much any
two-point sling, there are different mounting options, so come back when you get a rifle and we can give more specifics then.
Red Dot Sight:
Romeo 5, they're a good deal for the price and they just work. You can usually get them ~$100
Low Power Variable Optic (LVPO) (These are typically 1x - 6x magnification):
Viper PST is mid-tier, Strike Eagle or Primary Arm's offerings if you're budget. I don't mind the differences between budget and mid-tier but some people do.
Iron Sights:
Just go with Magpul Backup Iron Sights (MBUS). They're decent value for the price.