New Setup -- Your Thoughts................

Good afternoon everyone.  I am pretty new here and some what new to networking so forgive my ignorance on some of these questions.  I've just been assigned the wonderful task of designing and implementating a new project that will connect 4 different sites country wide.  Here is the concept.  There will be a main datacenter that will be the file repository of all the engineering files from all 4 sites (acutally another topic in the storage forum :) ).  Each site will have its own file/application server that the companies will use to analize their own engineering data.  Once the analizing is done, then the data will need to be replicated back to the datacenter for storage and future analization/collabration.  One thing to remember, is that each company has access to the network already and has been assigned different IP's  So, the stump factor here is how do I setup the network and/or servers to accomplish this?  Would I have the datacenter server act as the AD server and then just have the other servers join the domain through their existing network even if thats possible?  Or, do I need to buy a class C lets say or a block of IP's so that all of the servers are on the same ip string?  The other stumper is that each site will have 4-5 local computers that will need to access the local server.  So do all of these servers need AD loaded on them and the clients then log into their domain?  Its almost like a AD question but need help on the networking.....Was so use to the PDC / BDC thing and then MS has to go and do this to me :) ... tks everyone in advance.... hope this wasnt too stupid  :)...