it works best if defused.
You wouldn't want it going off unexpectedly would you?
Seriously - give serious thought to the 150mm Sigma lens - see many previous posts on this forum.
As far as flash is concerned, with a shortish lens, the on-camera flash will be useful (it works even with a 180mm lens on a 30D, but I think the flash sticks up a bit higher on this).
I have the amazing MT-24 EX Twin Flash - but I find it hard to justify the cost! Cheaper ring flashes are available, but give a wierd flat lighting.
As for extension tubes - the shorter the lens, the greater the effect.
Lots of decisions to make here - it's one of the things that makes photography such fun - deciding on kit.
Good luck.
Roger Harding