We did not need the SUV on the paved roads between areas, but did need it when visiting restaurants or tourist attractions (road into zip line, road into rain forest, etc). Some of the dirt roads off the main roads in La Fortuna were quite rutted.
Contact your charge card customer service before leaving home, as your charge card may cover part of the car insurance. There is a mandatory national insurance ($10-$15/day---can't recall now) but I did not need to purchase the additional liability insurance. I have a fairly basic AmEx and it covered the excess car insurance. (There are six countries AmEx won't touch; Ireland is one of them so driving in CR is safer than driving in Ireland.)
I would definitely rent the GPS also. Maintains harmony inside car on long family drives. I did not look into purchasing CR maps for use on my GPS so we just rented the GPS instead. You may find it less expensive to download CR maps to your own GPS unit, if you own a free-standing unit.
Edited: 10:59 pm, April 27, 2012