What is the minimal power level dBm, that can be used as LO after amplification? -40?

For example, i can amplify −40dBm power signal up to 0dBm, then use it as LO. Isn't that a problem that there are many transmitters with similar frequency. Can metallic housing save my amplifier from receiving unwanted −20dBm signals, that can excite on band pass flters/etc elements of microstrip lines?

A metallic housing should provide at least 60dB isolation against interference signals regarding to standard assumption.In the other hand jamming signal levels are pretty low in the air so -20dBm is not a realistic level at all since you don't work just beside of a Transmitter or Basestation..