wow. lots of questions. if you can, search a bit. i will try to answer you questions as much as i can.
you are doing alright as far as equipment goes. i believe there is a seller on ebay that is selling 4 kegs for 75 shipped, which is a good price. general rule of thumb is to get more kegs than you think you may need because you will end up using them... (right now i have 25 kegs). this is of course in relation to your budget.
tubing: you need two types, one for gas, one for liquid. for gas, use cheap vinyl or pvc tubing, 1/4'' thin wall is fine. do you have a LHBS? they should have it there. if not, you can get it cheaply for 11 cents a foot at USplastics, and 14 cents a foot at mcmaster, i believe. you will also want 3/16'' ID 7/16'' OD smooth bore beverage line for the beer line. again, your LHBS should have this. if not, i'm not quite sure which of the multide of options from USplastics and mcmaster you could get, but choose wisely if you choose this route. you will also need flared QD's, don't get barbed. you will also need to get clamps (either worm - easy to get, or stepless oetiker - superior, but a bit more expensive since it is difficult to find locally, you can get them from mcmaster or ebay).
the QD's you get depend on which style of keg you get. most likely is ball-lock, in which case just get the flared (MFL) ball lock Quick Disconnects from your LHBS or online, get equal sets of gas and liquid (meaning as in buy a pair at a time, don't just buy a bunch of liquid or a bunch of gas separately).
best option for splitting co2 is to use a manifold with cut offs and check valves. cheapest option is to use a 'Tee'.
i recommend check valves regardless if you value your regulator. if liquid backs up into the regulator diaphragm, it can likely be destroyed. i bought my inline check valves from USplastics for 1.00 a piece plus shipping.
you can also put the co2 tank outside of the kegerator if you want through the use of a bulkhead. you can either create one yourself with parts from Home depot or mcmaster, or just buy one online.
as for cooling, it depends on your setup. i think it may not be worth it trying to cool it, and at most have a fan circulating the cooled air from the fridge in to the tower, but it may not even be necessary.
also, properly balance your kegging system.
are you using a chest freezer or a top/bottom fridge? if a chest freezer, make sure you get a temperature controller, and if you are using a fridge, then make sure you know the temp of the fridge area so you can set your volumes of co2 properly. if using a chest freezer, i recommend a love digital temp controller. i bought mine at cole-parmer
so i guess to try to itemize it, you need:
1/4'' tubing (get a roll of 50' or 100' if possible)
3/16'' thick walled smooth bored tubing (get as much as you need to balance with, get a a roll if you can afford it)
clamps (worm or stepless oetiker)
QD's - as many sets as you plan on having for your taps, plus a set or two for carbonating (if you can afford it)
manifold or Tee
bulkhead (if you want co2 outside of fridge)
temperature controller (if using a chest freezer)
also, you should get keg lube, as well as extra gaskets and poppets. they will inevitablly fail on you, so better to have a backup handy rather than go desperately to your LHBS looking for the part you need.
if you don't have faucets yet, i would recommend forward sealing faucets, either the perlick or the newly labele shirron (originally ventmatic).
many of my extra recommendations come only as secondary as you have the funds. it has taken me a year or so to amass what i have now, and that was a lot of craigslist scouring, but i was able to find some amazing deals.
you may also want to invest in some BLC (beer line cleaner) to clean the beer lines and faucets.
edit - forgot to give you some online stores, in case you don't have a LHBS
i live in austin, so i have to support my awesome LHBS: www.austinhomebrew.com
there is also morebeer: www.morebeer.com
that should be a good start for you. haha.