Your bike generator puts out 3 watts nominal at 6 volts and .5 amp. The Halogen light you want to run that your buddy offered is probablty close to 60 watts or 5 amps at 12 volts. No way that you can use the bike dynamo to charge the battery for much run time. For 1 hour of running if using a lead acid battery you want 10 amp hours of capacity as you do not want to run a lead acid nbattery down much below 50% if you want decent life from it. Be heavy as hell. Two hours run time would be a 20 amp hour battery.

Spend the money on something like the Magicshine lights as they are a practical and lightweight solution. What you are thinking of is neither practical nor will it be that much cheaper by the time you purchase batteries, a charger etc.

The LED flashlights tend to have beams which are too narrow for mountain biking off road at night. For that you want a relatively wide beam so you can see branches etc.