I have heard conflicting information regarding the issue of paying ZUS on an umowa o dzielo. Is it necessary?
It is not necessary if you work on umowa o dzielo and are not in an employment relationship, see Art. Paragraph 6. 1 point 4 and Article. 7 of the Act of 13 October 1998 on the Social Insurance system.
However, if you do go down that route, it's a good idea to arrange health care cover (as you will have no access to the state system).