The dock no question as it's made to be a mobile workstation when combined with HP's Cloud Virtualisation to run Win32 apps.
You can of course use other docks or adapters but HP's dock is recommended for the ease of use as it's tailor made for the phone.
In regards to HP's support, I feel that as an enterprise device the support should be top notch however if you do run into issues there various avenues one can proceed with to get assistance -
https://www.hpe.com/uk/en/contact-hpe.html
Call any number and ask for device warranty :).
I cannot personally say from experience if they are good or bad, can only speak objectively on the matter taking into account the amount they have poured into the device and the push to get it into the enterprise sector.
I would say try the phone out at carphone warehouse and see how it feels for you.
I'm looking to head over to a store this weekend or next and checking one out myself.
If I was to get the phone I'd get it with the dock however that's just me others may disagree so to each their own :).