I am preparing to leave my last Windows PC (XP) for good.

Current situation:
On the above-mentioned XP PC I am using:
- Outlook for Calender, Tasks, Contacts, Notes
- Thunderbird for email
- iTunes for Windows to synchronize Outlook with my iPotTouch.

Email remains solely on the PC.


Future situation:
- SW installed on new PC: Linux Mint 17.3, KDE
- OwnCloud running on my own server

Now I want to replace Outlook & Thunderbird by Kontact, etc. In detail I consider to replace:

-Thunderbird by Kmail
-Outlook adresses by KAddressBook
-Outlook calender by KOrganizer
-Outlook tasks by To-do list in Kontact
-Outlook notes by Notebook or PopUpNotes in Kontact

Questions:
- How good is my choice of each KDE counterpart for each activity I am doing currently in Outlook and Thunderbird?
- Are there better alternatives in KDE or Linux in general to replace my Outlook/Thunderbird functions?
- What needs to be used and done to import all data into the considered KDE applications? (Should you propose other application then, what needs to be done and use to import data?)
- What application do I need to synch all - but not the email - with my iPotTouch
- What kind of smartphone could possibly be used instead of my iPotTouch and what application is needed to synch dat, except email?

I don't to use a public cloud storage for my data.

Thanks in advance for answers to my questions and any additional comment!

Kontact is the primary Personal Information Management suite for KDE, and it looks like your mappings are good.

You are certainly free to submit requests through bugs.kde.org for features you may be missing, or to look around and try out other options including Thunderbird (is available on Linux and usable on KDE), Trojita, Claws, Geary, etc.

Each should have mechanisms for importing (and exporting) of data (usually under File -> Import), as well as connecting directly to supported cloud services like OwnCloud.

I have no experience with i* devices and cannot comment on syncing options. Though if all of the data is on OwnCloud, there are apps available to connect and sync directly with your OwnCloud instance.
For Android devices, there is also KDE Connect which has a lot of nice device-desktop integration features.

Thanks for your explanations and especially for the hint KDE Connect!