CPL is a form of gradual filter.

Hoya HMC is not super thin, the super thin hoya HMC is the digital version. Beware that these filters are not stackable and does not have threads out the outer ring. I use SHMC pro-1 on my 80-200 with screw in metal lens hood.

Thicker filter does NOT cause ghosting, they can cause vignetting if you stack them up. CHEAP filters cause ghosting. The only cheap filter that has worked well for me is the tiffen haze-1, don't bother with cheap HOYA filters, all the non SHMC versions have relatively lower flare resistance. More importantly, because they are only coated at the outer side, if there is too much distance between the back side of the filter and the front element, reflection in that space can cause severe flares.

If you want ND that comes with bracket holder, you will need to look into cokin system. Again, if you intend to stack filter, get filters that are larger than what you need, that way the filter frame doesn't get into the FOV. I use step up ring on my lenses when I intend to stack them...you should see how l33t my 50mm is when I have it stepped up to 77mm with a CPL on it

HMC/SHMC hoya CPL will cost you an arm and leg....those run a few hundred RM.

BTW philip, last picture is teh l33t