our guests have always had good luck with crystal rental, hertz, jabaru, budget. you can find them all here.http://www.travelbelize.org/trans.html
check before you go to see if anyone knows if the bridge at Kendall is washed out or not. it washes out with each big rain.
once you leave the pavement at the peninsula junction, it will be a dirt/mud road for the next 20 miles. 4x4 is not a necessity, but most rental places insist you get a 4x4 once they find out you are going to Placencia.
another option is to fly and rent a car once here.