Hi Alex!
No experience with Canon digital gear, and still new to the 645D, so I cannot really address your first three questions. As a hobbyist returning to Pentax (again), I have an opinion on the fourth.
Originally posted by alex3985 4) Last but not least I would be interested in your opinion wheather this upgrade makes sense to you or do you think I just went nuts here?
Why not? As mentioned above, enthusiasts/ hobbyists and the like spend similar money on full-frame/ flagship bodies/ top-notch glass. No reason why you should be excluded from using a 645D if you can afford to do so.
I don't know about you, but in my case the joy of photography extends to the gear. For me, photography isn't
just about the image - it's also about enjoying a hobby that has proven to be a major source of relaxation and stress relief, and part of that enjoyment comes from using gear that I find fun during use. The 645D ticks that box. There are some other reasons I went 645D instead of something like a D810, but the enjoyment factor is my most prominent reason for gear choices these days.
I recently took advantage of the Adorama close-out deal and picked up the 645D + 55/2.8 kit. Still thinking about lenses for later down the road. Like you, my primary use for the kit will be landscape, so I'll be following this thread to read members' responses to your first three questions.
Enjoy!