How a SA is usually used, is selecting an appropriate attenuator to knock the power down to a level that is safe for the input of the SA. In your case, it is < 50mW... I would stay < 20mW. If I was interested in viewing a 2W transmitter's output and harmonics, I would use a 20dB pad with at least a 5 watt rating.
As for viewing a SSB signal, the stability of the oscillator is adquate for that task. It probably isn't good enough to be able to measure frequency precisely, but for most everything else it should be okay. I see no need to ovenize the oscillator. Anyway, ovenizing the oscillator isn't as stright forward as it seems.
As for your input, I would drop the filter (and the 50Ω resistor) and replace them with a 1-dB pad. The pad will help improve the match between the input and the mixer. It is easily calibrated out, so there will not be much of an impact on measurements. It is the engineer's responsibility to know the limitations of his equipment and how to compensate his measurements to account for those limitations. For instance, if you are looking to measure the output of a 90MHz transmitter and its harmonics, you will know that the 2nd harmonic on will be OUTSIDE of your equipments capability, so you will have to take steps to filter out those harmonics prior to the input to your SA. If you don't, you will see image products IN YOUR PASSBAND that will try to confuse you.