If you don't need a ton of storage a single top case can hold a helmet or 2 depending on the size you get. You can always install side bags later if you need more room. The Givi ones are very easy to remove and install when needed. They do have a warning to not exceed 120km/h. Not sure what will happen if you do, I have not tested it myself.
Well after some deliberation, decided to go for an Ogio tail bag. Large enough to throw in lunch, pair of shoes, water bottle, and starbucks gift card... without being too bulky. Seems good so far, kept my stuff dry when I got caught out by the rain the other day.
Bag attaches via 4 straps (2 front, 2 rear). I have my front straps looped around the frame, and the rear straps looped around cargo hooks below my seat. The straps are designed to go around frame, hooks, or under the seat, depending on what your bike is like.
The front straps seem to loosen up ever so slightly, probably from where my butt/bottom of jacket move it around.. the straps at the back keep it locked down secure. This particular bag also has a rain cover that flips out which seemed to work well the other day when I rode home from work in a downpour. It's also compatible with the Ogio sport sidebags, whose straps go through the bottom part of this tail bag to keep everything locked together. I don't own the sidebags so it is difficult to show, but the guy at Holeshot showed me, and it seems to work.
One thing I've noticed is that you always want more storage than you have, no matter how little you think you need in the beginning. In this CASE, more is better
Did not George Carlin have a whole skit on that subject