For my Alfines (8 spd).
1x crank (1x LX octalink, 1x Alfine)
1x ring (1x 32T SS, plus bashguard to make look neat, 1x 44T Alfine)
1x sprocket (1x nexus 18t – nexus is dished so you can use to space out the chainline, 1x Alfine 20t)
1x chain (2x whipperman anti-rust SS 9spd chain)
1x fitting kit (well 2).
1x Shifter (2 again), comes with gear cable and end stop.
Di2 is the electric shifting version, lots more money for something I’m not sure is needed, Di2 is good when it auto adjusts the front and rear mech, but on what is effectively a singlespeed I’m not so sure.
Depending on your frame you might need the chain tensioner (I have an EBB on one bike and trackends on the other so no need), a vertical dropout would need a tensioner.
I run my mtb at 32:20 which is about 3rd from lowest up to about top middle on a regular mtb setup, I get up about 95% of stuff I would on the FS, it’s more of a plodder though and you sit in a gear rather than looking for incremental gains.
I run my general hack bike at 44:18 which is a little high for towing duties offroad, but gives me about 28mph at a comfy 80 candence so it’s rare I spinout on the flat.