Hey ningnangnong - Not sure what black post you can use as it needs to withstand sliding and clamping, doubt anything painted will last. Probably better to track down a titanium post which I understand is darker grey than the steel shiny pilar that comes as standard?

Black mud-guards- no idea but have you looked at other folders? Reckon you will have to mod something to fit. You could buy car spray paint and coat your current guards but doubt it would last any time at all.

Shifter - there are lots of discussions on this. Depends what you are looking for, one shifter to do all on an S6L seems difficult but there are options to change to grip shifts, thumb shifts, trigger shifts. Have a look at Sturmey Archer's website.