Second, it needs to have endurance so that you don't have to send a ship out to replace it.
Third, it needs to be of small enough size or distant enough from the warp point that it will not generally be discovered by ships that pass through.
Like Starmantle, I find your third specification needless for the role, a clear example of feature creep. But the second one is just meaningless - since the big engine changes, buoy endurance is automatically infinite:
Missile sensors must be powered. The power requirement for any sensor is equal to its 20% of the sensor strength. So one missile size point (MSP = 1/20th of a HS) allocated to an EM Sensor using a base EM sensor strength of 8 would result in an EM sensor strength of 0.4 (8/20). This would require a reactor with a power output of 0.08 (0.4/5). The reactor space is allocated automatically but displayed as if it was added by the player. Ship-based sensors do not require reactors as their needs can be met from the general power generation of the ship. On a per HS basis, passive sensors are much less powerful than active sensors at the same tech level, which means missiles will require less reactor space per MSP of passive sensors compared to active sensors.
There is no longer a separate 'buoy' category but you can create the same effect by designing a missile with sensors and no engine. The necessary reactor space will be added automatically. Missile reactors have unlimited endurance so there will no longer be a need to replace buoys every few years. While unlimited endurance is unrealistic, modern naval reactors have a service life measured in decades so this is a compromise between realism and a desire to reduce micromanagement. I may add some form of failure during very long term deployment - a failing IFF system on a mine could be entertaining - but I haven't decided yet.