Useful accessories that I added to my 7.2:

1) Decent quality lights. I do a lot of pre-dawn rides, and for any daytime rides in traffic, a front light in invaluable when approaching intersections. Rear lights from dusk until dawn are pretty much essential if you don't want to end up as road kill.
2) Bike computer. If you want to go faster and farther it definitely helps to know how fast and far you are going in the first place.
3) Saddle bag. Self-explanatory.
4) Water bottle / cage. Essential.

Useful upgrades:

1) Grips with integrated bar ends. The standard grips didn't do much to soak up vibration - though you may have better ones on the 7.4. On longer rides, the additional hand position is VERY welcome. I find the bar ends also work a treat when out of the saddle and also on tighter turns.

2) Clipless pedals / cleated shoes. More efficient pedal stroke, feet always properly positioned on the pedal and never slip off at just the wrong time.

Not sure why you would want to upgrade cassette / crank / stem, but then I have never really understood upgrading as a sport unto itself.