As I'm using a lot of probes missiles for quite a long time, I can tell you that...
- The fire control size is not important at all : the smallest you can build is far enough.
- You cannot indeed directly target a body. But you can target a WP that you convenientely set on the body you plan to survey. In the system map, click on the planetoid you want to survey to "select" it, then click "add waypoint on last position" (or something like that - I cannot run Aurora on this crappy laptop :s)
- You will need some well designed 2-stages missiles for this purpose. The first stage will be the launch vehicle, with only fuel and engine, and the second stage an engineless buoy with the sensor. Be careful to set the separation distance at 0 instead of the default 200,000 km
- Be patient. Very patient. Geosurvey buoys are slow as hell. I phased them out a long time ago in favor of swarms of geosurvey fighters. However, I am still using a lot of Sensor Drones for scouting potentially hostile space. Here are my current designs :
Name MRSD-3 "Glimpse" LRSD-3 "Peeker" B/AS-3
Missile size 2 4 1
Engine size 0,4 1,2 0
Engine multiplier 1,25x 0,80x N/A
# of engines 1 1 1
Missile WH Size 0 0 0
Fuel size 0,6 1,8 0
Agi size 0 0 0
Sensors 0 0 0,781
Reactor 0 0 0,219
Missile WH Size 0 Size 0 Size 0
Speed 6300 km/s 6000 km/s 0 km/s
Range 2220,49M km 21355,23M km 0,00M km
2nd stage 1xB/AS-3 1xB/AS-3
Those are specially designed for my scout ships size 4 launchers, I am pretty sure you can do far better with ICBM sized missiles (the research time for those will be a pain however)