I'd go with the walthers kits and just buy them over time. Thats what i had to do for my layout. it was until last june that i even had all the basic structures i wanted for my 4x8, and at that point i had my layout for 3-4 years. I didn't even have a job until right after i got the last kit.
Here is what i would suggest. You recently said you were redoing your layout, so here are two top notch suggestions-
Loads in Empties Out
This is an awesome way of running a railroad. basically, you'd have a pair of industrys that take a unit train of something, (like coal mines and a power plant). each industry would have a loads in track, and an emptys track. However, the trick is that these tracks would be SECRETLY CONNECTED! so when your BN crew backs up an empty coal train under a flood loader, the empty cars go through the hidden tracks and come out the other side of the building that "houses" the unloading machinery. at the same time, a full train of coal should be at your coal loader, and you pull that train out to the power plant, and back it in its "loads in" track. you grab the empties that are at the plant, and bring them back to the coal loader, starting the process all over again.
Its one of the best ways to keep your layout busy.
Magic Pan Bakery- The versatile kit
Magic Pan bakery is another good kit to start as your "base" for a model railroad. Magic Pan needs loads of grain, and it needs insulated box cars/reefers and other things for it to both make goods, and ship them out. An addition to magic pan would be a tank car track, as alot of places like these need shipments of things like corn syrup, which are also shipped by tank cars
SO where does Magic Pan get grain?
From the ADM grain elevator. So you would have this kit on the corner of your layout somewhere with some grain hoppers, but also a loading track for box cars (surprisingly, alot of grain is still moved in bags by boxcars), and another track for heavy machinery that is sometimes shipped by rail ( these last two don't need to be big)
What about the Boxcars?
get a warehouse. Nu-line makes big warehouses (then again, they are modular, and can be built to a size you like), and walthers makes Lakeville Shipping. warehouses can ship just about any kind of freight commodity put in a box car, so box cars and things can go back and forth. It doesn't need to take to much space up either. You can probably make the warehouse a "backround building" kit, to save space.
What about the Cornsyrup?
You don't need a huge facility for this either. One prototype cornsyrup place in NJ that i've seen is literally a small warehouse building like ST plastics by Great West Models, only it had 2 indoor railcar loading areas (but you only need one in your case anyway!), and was a bit longer. luckily, GWM's parts are also modular, so you can buy packs of their warehouse walls and kits to make a stucture you like! With two tracks for loading tank cars, you can bring the tanks cars back and for as magic pan needs them. Here again, you can make this a backround building kit, being long enough to enclose a couple tank cars.
You can try one or both of these ideas, and it will fill your layout with operations. If you plan right, none of these industrys will take up to much space, and you can run a vareity of eqipment like tank cars, boxcars, Flat cars/gondolas (machinery loads for the factorys), covered hoppers, and Coal gondolas, to name a few.
I hope i've helped you get some ideas!
the kits mentioned here are-
ST plastics
Walthers Model Railroad Mall -- product information page for 24-106
ADM Grain Elevator
Walthers Model Railroad Mall -- product information page for 933-3022
Magic Pan Bakery
Walthers Model Railroad Mall -- product information page for 933-2915
Nu line warehoue (keep in mind you can make it smaller!)
Walthers Model Railroad Mall -- product information page for 533-8710021
You can also get a backround building kit of this building, but i can't find it. It is much smaller than the Nu-line
Walthers Model Railroad Mall -- product information page for 933-2917
Tri State Power ( you can still find this kit despit it being retired)
Walthers Model Railroad Mall -- product information page for 933-3055
Wester Flood Loader
Walthers Model Railroad Mall -- product information page for 933-3089
Just save up your money and get them one at a time. trust me, you don't want to do this all at once!
Space is also not as much as a proble. Your layout should have enough space if you plan it right.