For a serious work like you described I would certainly recommend to select Agilent, Anritsu or Rohde&Schwartz test instruments. All produce vector network analyzers capable of operating up to and above 20 GHz. All are expensive but this guarantees a long-term calibration accuracy.
There are also opportunities to rent such instrumentation, to use it over the time needed but not own it. Renters are responsible for calibration and possible repairs.
If you make a top-notch products, you need the top-notch test instruments. This is why good products are not cheap.