Rules, upon rules, upon rules...

So, I'm purchasing 2nd hand stuff left an right, getting some STEALS in the process!!

However, my mind is reeling on the rules... Which type is a unit? how many can I take? Do I give them upgrades? Are they worth it? Why can't I take a Land Raider Crusader, even though we've had how many Black Crusades!!?

And when you look up a unit or formation or detachment, you get to the "They can, unless, whilst, with exception of, have to adhere to", yada, yada. Oh my...

My mind is getting saturated with things to do, and it's hard to keep a concise path to follow.