Proper design of the PWM low-pass filter involves a few tradeoffs and is not trivial. I recommend you read this first:

http://www.tonnesoftware.com/appnotes/pwm/HamPWM.pdf.

As Wayne suggested, you'll need to build the RF stage and operate it at your intended carrier level in order to determine the filter load impedance (DC voltage divided by current).

Then use this program to design the filter

http://www.tonnesoftware.com/elsie.html The Student version is free and allows up to 7 network components which is sufficient for your design. Like Wayne, I felt guilty and bought the Professional version after repeatedly going onto Jim's website. I forget what it cost but it wasn't expensive.

In Elsie start with Inductor input lowpass topology Butterworth family. For 3dB Bandwidth use your chosen cutoff frequency. In general the higher you make it the lower the component values but that involves the first tradeoff. You need to consider your switching frequency and to what extent you want the filter to suppress it. In my case the switching frequency is 100kHz and I chose a 15kHz cufoff.

Next put in the number of inductors and caps under Order (mine is 4). Finally put your load impedance in under Input termination and select the Schematic tab. You'll see that your load impedance will appear both as the load and the source impedance. The source impedance is not accurate since the modulator is essentially a voltage source and its output impedance is very low. Use the Edit tab to change it to a low value. I made mine 0.5 ohm.

Now select the Plot tab. On the left side click on Input impedance;transmission and observe the filter response and input impedance curve. You'll see that there is an unwanted bump up in the response at the cutoff frequency and probably a dip down in the input impedance. At this point you'll need to use the Tune Part tab to play with values to both lower the bump up and smooth out the input impedance curve. This tuning procedure may seem like cheating but it's what Jim recommends.

As a final check click on Analysis/Transient and under Waveform Type select Squarewave with a Squarewave frequency of say 1000 Hz. If you have tuned everything properly you should see minimal ringing on the squarewave. My actual filter squarewave response has just a bit of overshoot and exactly matched the simulation.

As a final tweak I added a capacitor in shunt with the second inductor to produce a deep notch at the switching frequency.

This all sounds complicated but it was very educational and great fun at least for me.

Rod