Don't worry about the cost of the ADS, the credits you recoup from the missions will repay over and over.
Remember to add the ADS to a fire group (right hand menu) and fire it off as soon as you enter the appropriate system.
You then see the target planet in your left hand menu, or in the systems map (as you prefer)
Fly towards it.
When you get close, the mission location, called search area, or search base area (something like that) will show up in your menu. Fly down to it and follow it as it moves around until you get the final fixed location and a visual.
Fly to within about 300 m of the base (too close and you won't be able to work the data unit), land and drive in your SRV for the remainder of the distance.
Target the data point (Which activates as you approach) in the turret of your SRV, press your "download data" (which I have set to secondary fire button of the turret in my fire group menu) and you collect data and complete the mission. Drive back to the ship. You will get a message to say fly back to base.
Fly back to the source base that gave you the mission and collect all the lovely credits
Rinse and repeat. Also fire off the ADS every time you enter a new system to collect extra credits for discovering planets etc. And wonder how you ever survived without a ADS before
Edit - ensure your data is not full of other junk .... it's easy to exceed the data limit and be unable to complete the mission ... I usually trim 20% off downloaded data for which I have more than 25 or 30 units.