4-link, 3/5, 4/6 questions and pics

[Puglia10]

I think Im going to be buying my friends 81 c10, regular cab, short bed. He has a 4-link for it and a 3/5 lowering kit with spindles and a flip kit. The entire truck is taken apart, every piece...On to my questions (never lowered anything before, bear with me.)

1. Is the 4-link worth the 200 dollar price difference in buying the truck? He said it was $600ish, never used, powdercoated body color (red) What exactly is so good about a four link?
2. Can I go lower easily, say I want to change to a 4/6 or something?
3. I plan to do a c-notch while bed is off anyway. Just get a bolt it? or does it depend how low I want to go? (will stay static)
4. Anything else I should think of doing while the frame is by itself?

Feel free to post pics of different drops and suspension set ups!
Thanks for any help! Sorry for the (probably) dumb questions, new at this.