I am sure some guys on here actually own Hobies and regularly fish off them so they can probably help more.
I was very interested in these at one stage, and constantly used to use my friends on. My friends one however was the Hobie Adventure, they are longer in length and narrow. More suited for the open ocean, but much less stable than some of these models.
To the best of my ability, some answers to your questions;
Do i necessarily need a trailer for one of these or can i attach it to the roof via roof racks?
We always attached them to roof racks, I have not really seen anyone of them towed yet.
Are they user friendly; ie: easy to use on the water, easy to launch, plenty of "safe" storage?
They are a very easy system, I have heard these models are very stable too and yes there is storage. Usually I have a dry bag that I keep my wallet, etc in and that way if it goes overboard for what ever reason it actually floats.
What Sounder/Fishfinder would be suitable for use on this kayak.
Most models will fit as long as you have a battery pack, there are videos on youtube to assist you.
Here is a few links to youtube videos that can help you more.
http://www.youtube.com/watch?v=gvnPUJsMEto