How to calculate the impedance of a rectifier?
Is a DC-DC converter necessary between the C3 and sensor node?


These diodes have a reverse breakdown voltage of about one volt, so even if you had a strong RF source, you'll get only about half a volt output for your sensor... diodes have to deal with peak-to-peak RF voltage. If your sensor requires more than half a volt, then a DC-DC converter will be needed.


  Should I consider the impedance of the wireless sensor node? If needed, how to measure the impedance of the sensor node?


You show the sensor as an equivalent resistance (this makes sense, because it will absorb power). It is up to you to characterize this resistance, we are told nothing about it. The L1, C1 matching network will need values to deliver maximum power to this resistance.


  How to calculate the impedance of the rectifier?


This is rather difficult tp determine, because diodes are non-linear devices. Diodes have a minimum resistance, this one's data sheet says it is 20 ohms. Its "video resistance" is about 5000 ohms, and might represent a maximum. At zero bias, its dynamic resistance is even higher, and depends on temperature. In addition, both diodes do not conduct all the time, which makes their effective resistance higher still.

A SPICE simulation might help choose appropriate L1, C1 values for the matching network. The diode model would look like:

.MODEL Dsms7630 D(IS=5e-6 N=1.05 RS=20 CJO=0.14e-12 TT=1e-11 M=0.4 EG=.69 XTI=2 FC=.5 BV=2 IBV=1e-4 VJ=.34)