When I started collecting train LEGO I soon relized that I will need some general brick collection to be able to build enviroments for my layout. At the time I only had technic parts, so I needed a complete new organizing system. It seemed easy to buy a mixed used collection on eBay. In avarage the price goes for little more than 10 euro per kilogramm. I bid and won a total of more than 30kg.
Next is to wash all the pieces and organize them. This is the part which takes lof ot time. I'm still far from finished. I'm not even sure my organizing system will be final. Compared to the technic I can't store everything on my desktop plastic cabinets. I also need many bigger boxes.
As I go along the little cabinet boxes are filling up fast and I need to rearrange something and use bigger boxes. I'm just half way through, but at least I now have a feeling about the kind of parts I can expect.
Now I also realized that a lot of these parts I won't be able to use directly in my train city. The parts are usually come in the basic colors, but most of the advanced sets like Emerald train or modular buildings need special or shaded colors. This is why those sets look professional and life like. The basic yellow, blue, red, green colos are too vivid and saturated.
Never the less these bricks are still usefull for experimenting or hidden areas and I got also a lot of the special bricks and minifigs. It just takes forever to finish sorting...