I just got back from my second Ring Road trip to Iceland. I wrote a 5 part blog on my adventures along with some tips for travellers and photographers alike. The following links are not business solicitations in any shape or sort, just my experiences. I will freely answer any questions or talk about how a particular shot was done.
I rented a camper van and made the most of the nomadic lifestyle. April was the pre-season and the various sights were less busy than say June. There are many hidden gems beyond the obvious stops...some are better for sunrise, others for sunset. If you have less time, most of the winners are to be found in the South and East.
Personally I don't recommend a tour. It's pretty obvious how to get anywhere and see anything. Rock's book was very helpful to me, even on my second trip there.
My biggest trip expense was probably gas. I found that a 9 day trip was a bit much; A week was ideal for what I was doing.
Once you've seen Iceland, you'll be back.