I think there was a bug with the passive sensor buoys but I suppose it is fixed, didn't try it yet on 1.9.5.
You can make the buoy with at least 0.25 msp sensor. If you want both EM and thermal you should at least put 0.25 for each. It will show you the sensor range. No engines, no fuel. It will allocate some space for power plant itself. I like to think of it as mini solar panels.
From that point you have two options.
1. You can use only the buoy with a right sized launcher and go to the place you want to deploy the buoy. Then give "launch ready ordinance" command.
2. You can put the buoy as a missile's 2nd stage. The first stage have an engine and fuel, no warhead obviously. Now ypu have a range and you can fire the buoy missile with MFC targeting.
For actives, they are needed to make cluster missiles or mines. When enemy gets close enough actives will trigger the mine or just the first stage so it will either fly to the enemy directly or it will trigger the second stage through seperation mechanic and will break apart to launch clusters inside. If you choose second option you'd need an active sensor for each cluster piece and those also need engine and fuel to go to the enemy. It's like reverse of the sensor buoy design.
Edit: I designed a sensor buoy with 1st method anf it worked as expected. I deployed it in place as it has no engines. Only needed a very small mfc and size1 buoy and size 1 launcher.
Buoy has 0.35 em, 0.35 thermal and automatically 0.220ish power plant, no engine is checked.