varator in a vco

There are several references on VCO design from skyworks.
http://www.skyworksinc.com
For varator model, actually it could be found from most vendor.
And for the choice, the first thing is to ensure whether the usable capacitance range can cover the freq. band in your design and application. Then you should further consider the performance like phase noise, linear tunning range, tunning gain etc.