Thanks for your answers guys, really appreciated.
My work is almost studio/landscape at the moment so the 645D seems to be an ideal choice for me.
Here you can see some of my recent works, especially in jewelry where I think this camera could really shine.
Just read the comment from Mr. Chambers about supposed hair scratches in an inner element of a brand new 55/2.8: it could be a very unlucky one of a kind coincidence but I'm really curious to see how it evolves and wondering if future, newly produced, lenses will be affected by similar QC issues.
I've asked for more infos here because an user over FM pointed me at some past issues with AF motors in some Pentax 35mm format lenses.
Well, afterall it seems I need to start to build up another lens line up.
BTW, this one would be my first Pentax! LOL