I got a DID chain and sprocket kit from wemoto last time. You can probably save a few quid by buying them separately on ebay. Get an x-ring DID chain, and JT, Sunstar or Rethnal steel sprockets. Do not get aluminium sprockets, the weight saving is not worth it and they wear out quickly.
As far as teeth go:
Rear More Teeth, or Front Less Teeth = Faster Acceleration, More Revs for a Given Speed, lower top speed
Rear Less Teeth, or Front More Teeth = Slower Acceleration, Less Revs for a Given Speed, in theory a higher top speed but you will run out of power first, also in theory a better fuel economy but in practice at speeds over 30mph the effect is negligible due to wind resistance being a far bigger factor