Several months ago I planned a similar trip... but ended up scrapping my Parati plans in favor of additional time in Buzios. I just didn't like the idea of spending so much time in a bus while only in Brazil for 10 - 11 days total.
On my next trip to Brazil I'll be there for much longer. Thus, I'll attempt to visit Parati for a day or two. However, Buzios is definitely a "must-see" for me whenever I'm in Rio.
Instead of hiring a private car/driver through my pousada (expensive) or renting a car (expensive and dangerous), I opted to take typical transportation... the bus. We bought our tickets from some type of travel shop in Rio. I'm sure it wouldn't take you long to find a similar shop.
I think travel time to Buzios was about 3 hours each way and the cost was roughly $10 - 12 USD each way. Pretty cheap! Just be careful at the Rio bus terminal. It's a zoo and I carried my small backpack in front of me per the recommendation of my Brazilian friend traveling with me. (Travel light b/c there is no need for lots of clothes when you're mostly wearing beachwear and other casual clothes).