I heard many times about a hadith saying that if one recites ayatul kursi after prayer, nothing stands between him and paradise except death. So first I wanted to know is this hadith authentic? If it is, does it include people who sin regularly (as in its very hard for them to not sin)?

This hadith has been narrated in our sources and it is widely accepted.

It includes people who regularly sin as long as:

1- They die with faith (sometimes excessive sins make one lose faith at the moment of death)
2- One still respects Allah and tries to repent (not one who is careless and negligent).