Hello
I searched this forum but did not find answer to this situation: I renewed my studio equipment and am in process of installing Dorico Pro 2 (Upgrade from Dorico 1) to a brand new iMac Pro (which does not have Dorico 1, as I run that on old 2005 iMac) – so does the Upgrade Installer install everything (Halion sounds etc.) or should I install Dorico 1 first to this new iMac Pro?
Welcome to the forum!
Dorico 1 is totally separate. Just download everything from the Dorico 2 download installation package.
Thank you Dan for the info and for such a rapid answer on a Christmas Day! 
Assuming you were using the Soft-eLicenser on your old computer, the one thing you will need to do, lpaasikko, is use the ‘Reactivation’ button on your MySteinberg page to get a new Dorico 1 activation code and put it on the Soft-eLicenser on your new computer before you attempt to activate your Dorico Pro 2 update: the update activation code you’ve received will only be accepted if you have an existing Dorico 1 code on your Soft-eLicenser. If you’re using the USB-eLicenser then this is no problem: you can simply insert your USB-eLicenser into your new computer and everything will work out OK.