I'll answer the questions best to my knowledge, keeping in mind I am far from an expert here.
1. Which supplier should I go with? <- this is complete subjective, most companies offer quality parts and replacements.
2. Changing to disc brakes will my front wheels fit on the same way as on drums or will they be pushed out, I have 20x9" with 5" backspace? <- a 20" wheel will fit perfectly with disk brakes, essentially with a 5" back space.
3. If the front wheels will be pushed out, do I have to push out the rears as well? <--most kits are supplied with a spindle that pushes the wheel out about 3/4 of an inch (hardly noticeable if you ask me). CPP offers a bolt on spindle that keeps the complete sock wheel position.
4. Does the rear need lowering as well if I drop it further the front by 2.5"?<-- once again this is preference. Most people lower both front and back when lowering their truck, unless you want the rear to stand a lot higher. I believe the common 2.5" front drop would use a 4" drop kit in the rear. this page will give you an idea
http://www.lmctruck.com/icatalog/cb/full.aspx?Page=139
5. Do I need a separate lowering kit?<-- If you want to lower the rear, you will need a separate lowering kit for that.
6. Do I need a sway bar and what it does? <-- The sway bar helps stabilize the truck, especially while turning, I do not know if you "need" one in the rear of the truck.
7. Will the bolt size change as currently it is 6x5.5? <-- you can get a kit in any bolt pattern you desire. yes, 6x5.5 is readily available.
I hope this helps at least a little bit.