'easy' is very relative here. That is going to depend on quite a few factors. What I would do if I was in your position is install a sandbox environment the same way as you did before and find out all these answers for yourself. There should be older versions of the software on the download center that you can use as a base version to upgrade from.
As for downtime, I would test out the zero down time feature and see how that performs. If you can afford to take a downtime to do the upgrade I would look at trimming down the DB size as much as possible before hand. Maybe run the upgrade with no statistics and/or reports for example.
And as always, make sure to grab a DB and binary file backup before upgrading any system.