Do the Uncommon and up NPC's spawn on your BT at random? And is it helped by increased scanning?

They are random, you need some scan in order to see some of NPC's since they have cloak but you will be able to see everything with 600 scan. More than 600 scan is useless unless your trying to find the hidden npc's then you will need 2000 scan.

As far as scan affecting the amount of rares/uncommons there is no effect.

What would be a sufficient scan for all npcs in the range of lvl 75-150? Also, what are these hidden npcs you speak of?

1000 scan (and level 350) needed for XRP warp stations
though with attack and defense over 10K, and 65K required for a lock (drops for 10), and a random debuff of -50%....
most legions would do best avoiding them...

50 scan should be enough to keep finding Rogue Labpods and TO Troop Carriers. That's one Genetarr transmitter. I think you can get Dark Megacomplexes somewhere around that rank. They need 200 scan I think. That's all 4 genetarr transmitters

when you get to rank 200-220 you'll want to up your scan to 300 to get the XRP prototype then shorlly after up to 600 if you want to get a Shadin Hypergrid. Don't worry aboutt he XRP Warp Station for now. That needs 1000 scan.

There are 2 new NPCs which unlock mission chains. One is 1500 cloak and the other is 2000 cloak. Both are Extremely Rare so don't worry about these for now either. Think you'd need to be higher rank anyway.

So, 50 is your minimum. 200 is better. Once you get to level 200 or whatever you'll want to find those XRP Prototypes because they drop energy artifacts for your shiip. You'll need 300 scan to find them.

Unless you're in a legion with a good number of rank 300+ members, I would keep your scan UNDER 500 until you reach rank 300. Hypergrids and Dark Command Nexus which show up at 250 and 260 with 500 and 600 cloak (those numbers might be jumbled a bit as I can't recall which gets which). Those are a real bear to take down when you first rank into their range, especially if there are only a few in your legion that can see them.

Okay, thanks for the advice, everyone. It sounds like 200 will be my ideal scan for the next couple of months.