In order of importance:
- oil filters from Amazon and few quarts of dino oil - break it in properly! Read about it
here or many other place on the web.
- hugger to keep crap off the shock
- shop manual if you want to do your own maintenance
- sliders - saves paintwork if you lay it down in garage and your leg if you go down
- pazzo levers - it's not a bling, but serious improvement in control feel and precision (for me at least)
These are necessary to start with. Other you can do later - you'll find a lot of ideas here how to spend your money. For me it was dual Arrows (very much disliked the look of stock cans) and flyscreen (a little protection pushing up the cruising speed).
Have fun!