well... where to start...
I guess the scale you are aiming at is MUCH to large. Cut short on a single Company, those are enough work to clear, assemble and paint. It took about a year to complete mine. I don't want to offend you but it pointless havong such a large army and the time to create it should be spend outside... or anywhere else. Just building and owning the boxes is just the smallest part. I would just buy as many models as you can handle to build and then go on with the next, well i guess a decent and fast painter might accomplish about half a battle company and some vehicles a month (so about 50 models and some tanks), i assume you work for the money so you should not have infinite time a month.
-Start slowly... but keep painting (and assembeling), with the scale the potential painting styles like diping should come in handy (see fanatic army painter they got some decent tutorial, other products like stain or glaze. Building more models than you can paint is really frustrating and not that nice to look at.
-For the first time i'd start with about 1-2 squads a month increasing the number depending on time and skill. Buy new stuff only if those are really limited offers (time or number wise) or when you can estimate when the "old" stuff will be finished.
-Think of a good place where to stash the models, boxes with some DIY fillers (cardboard, foam) work fine, i guess so many models will not leave the house, so you don't need to spend to much money on solid cases (1 big case for each army but they are played i frequentliy and the size is not much higher 2000 - 2500 points except for may DA army)
- fluff wise the organisation was not that different than the "current" chapter organisation.
some decent information should be availible here: Lexicanum