I feel you are worried about the difficulty and quality of the roads, so I'll address that.
The main roads are well maintained asphalt and it is easy to drive to the most popular spots like Golden Circle, Vik or glacier lagoon.
There are many gravel side roads and in some of the areas they are the only option (West/East Fjords, including parts of the Ring Road), but they are quite well maintained as well. If you are a careful driver, you will be ok. Especially if you decide to go for a car with higher clearance and bigger tires. Unless you plan to drive deep into the interior, I wouldn't say you need to worry about very difficult driving conditions.
That being said, if you are afraid to drive on narrow roads that are not asphalt and have no idea on how to change a wheel in case of a tire puncture, you may need to be extra careful.
As for the bus tours. In my opinion having your own car is always better, because you can take your time and enjoy scenery on your own pace. But that is whole another discussion.
EDIT: Two useful links when planning a road trip in Iceland:
Road conditions (with web cams)