You need both, exactly as you said in your third questions. You'll get higher quality/value scans of planets, moons, suns, etc. with the detailed surface scanner.
You can upgrade the basic discovery scanner to one with a greater range for it's "ping".
You do need both. The detailed surface scanner improves the quality of your scans of unknown celestial objects, so you get more for selling your data. The Discovery Scanner is the "ping" device that locates the unknown objects in the first place. The basic discovery scanner has a radius of 500 Ls, the intermediate range is 1000 Ls, and the advanced scans the whole system.
the advantage of advanced being that sometimes systems are over 350,000ls
Can I replace my discover scanner with a detailed surface scanner?
or do I need both?
the discover scanner to scan the system, then the Detailed surface scanner to scan individual "Unexplored" locations?
u dont need both. All i use is the advanced discovery scanner.