I grew up boating
on that river as a kid. There was a lot of commercial
traffic back in the day and the river is navigable all the way into Belarus. Here is the list of all bridges from the government
It looks like you might be good with the mast
up until Kherson. I think there are some sailing/boating clubs in Kiev and Odessa and you can contact them for local knowledge.
The scenery is nice but not spectacular however there are several major cities along the river you might find interesting. Also there are 6 or 7 hydro dams you have to lock through between Kiev and the Black Sea.
As far as challenges, I would expect a lot of red tape
, language barrier and lack of infrastructure for private boats.