before i even install these 20" rims should i touch up on my suspension or brakes or etc...?

 

i think you would get more answers if you were a little more specific about this question. What year/make is your car? What are you looking for in terms of a finished product? How much are you willing to spend? What are you going to be doing with your car, just daily driving, tracking, or a little of both?

 

Yes you will need a drop...Tein S Techs will do it.
As for brakes, upgrade your rotors to slotted or cross drilled and also upgrade your brake line to stainless steel.

 

lower it! but you can do that before or after you slap the new wheels on...doesnt really matter as long as you get the camber back into spec asap.

i'd suggest lowering it first though. that way the springs can settle and you can get an alignment before the new rubbers go on.