Be careful when looking for caps, you can get over 100V if this thing is tuned correctly.
A typical value of the capacitor might be 350V, I used capacitors with a 600V working range. The trick is not to get the coil on resonance but just to one side. You will notice that when you introduce metal close to the reader it shifts the resonant frequency. Make sure you tune just off the resonance on the side that presence of metal does not put it on peak resonance. The bigger the coil diameter the greater will be the read range.