I've been using this Smart Weigh SWS600 Elite Pocket Sized Digital Scale 600 x 0.1g, Black for over 5 years. Haven't been shot due to a deal gone bad. Yet.
Cheap, small and very accurate
I've never used a knockbox, but I agree with the rest. Also:
A small, digital scale in grams. Besides the grind's coarseness, accuracy and consistency are key.
If you're making milk drinks, a pitcher. Milk poured to the crease in the middle is perfect for a (6 oz) capp.
A thermometer for steaming milk, if you don't yet trust your instincts (I don't).
A blind basket for back flushing and cleaning.
Caffiza for occasional deep cleaning.
Nice-to-have: A vacuum sealed coffee canister so that beans stay fresher a bit longer.
Our favorite cups: Cremaware
I just picked up this one http://www.amazon.co.uk/dp/B00GS8LWIW
It actually just arrived today. I played around with it a bit and it seems solid but I obviously haven't had much chance to use it.