Well for starters you need to help us help you, by finding some data on variuos parts you have, like what voltage do the dynamo put out at what rpm of the wheel (approx rpm), any idea on what current the dynamo will supply.
What is the voltage and current needs for the led headlight.
You ask about battery packs? but without the above information we can not give an answer.
Light sensor is the easy part, and a LDR and a opamp or comparator would do that, but we need to know the data on the led headlight so we know the current supply needs and the voltage.
Best you do some of the leg work, and then we can help fill in the gaps.
Just remember the greater the current load that is drawn from the dynamo, the greater the friction drag applied to the bike wheel, and hence the more energy required to pedal the bike.
The more power you extract from the dynamo will be like putting the bikes brakes on more and require you to pedal harder to over come the added friction.
Pete.