[Fallschirmjäger]

My S&W M&P Sport II has 16 inch barrel, want to upgrade the handguard and buttstock.

Using my S&W 15T as the standard....
Handguard is standard 'carbine/m4' length
Receiver extension (aka buffer tube) is mil-spec diameter as opposed to commercial.
If you want to replace the adjustable stock with a fixed-length one you will either need a spacer or a new tube in addition to a new rifle buttstock.