If you're planning to use a straight forward buck you're better off using a fan with as higher input voltage as possible, go for 24V or even 48V if possible. Although switching regulators don't burn up extra voltage in the same way that linear regulators do, simple buck regulators are more efficient when the output voltage is nearer to the input voltage, the only way round it is to add a transformer but it won't be a simple buck anymore.
You could adapt the circuit on
this site for 160V input, you'll need to play around a bit, I'd increase the resistor values and use a darlington for the driver and obviously make sure the transistors, capacitors and resistors are all suitably rated.
Also make sure your circuit is fused to smoething like 100mA.