exposure lights are the shizzle! Mountain biking products, like many things, vary widely in quality and I am now a firm believer in the Chris King component theory (makers of expensive but highly durable bearing parts). Spend a lot on top notch kit that will last years, and enjoy the savings over the long term.
Make the investment, buy decent kit. A head mounted exposure diablo light would be sufficient. With a bar light at a later date if needed.