I've booked 3 or 4 days in Buzios in early April (on the way thru to Curitiba).
What are conditions like then? I.e. what should I put in my golf bag?
What are the best beaches for kitesurfing off?
Anything / anywhere I need to avoid?
You should check the forecast a few days before you leave, than decide what to take.
As you can see in windguru http://www.windguru.cz/pt/index.php?sc=426&sty=m_menu
, from april till june the weather changes a lot.
If you get wind from NE or E you should go to praia rasa ( a kite 12 would fit most of the time), if you get wind from S or SW you should go to Tucuns or São Pedro dáldeia (it´s a lagoon) 25 min by car.
Since it´s only for 3 days, if the forecast doesn´t show at least 14 knots from NE or E , i wouldn´t take anytinng.
SW and S forecast usually comes with lots of rain and stronger winds (30 knots good for a 9).