Although this is more or less theory at this point, looking ahead I can see how my railroad will be completed in some distinct phases. In fact, there are different phases for different potions of the layout, but for this posting I will be focusing on the roster.
I'm primarily working within this phase right now. A big portion of this is research of both the prototypes and models. In the process of collecting and organizing the prototype info, I started making a binder of material along with entering a lot of data into the computer to make the key information easier to find. This website is an extension of that, because I figured I wouldn't be the only one that would find this information useful, and it allows me to access it anyplace I can get to the internet. A side benefit for me is that others have the opporutnity to "proof" my data when they visit the site. I've already had some feedback and corrections, and I hope that process continues.
Collecting modeling info is a combination of a few areas. One is identifying and acquiring modeling info about the New Haven Railroad in the hobby press. A side benefit of this is that I also acquire some info on other prototypes as well. The next area is identifying what models are/were available for the New Haven. This can be a little more complicated, but not nearly as complicated as the next step: identifying which models are the most accurate, and/or are the easiest to "correct." The modeling articles are helpful in much of this regard, but online forums and other modelers are invaluable. Again, in the process I end up with information about other roads.
Home Road Phase
I'm sure most modelers are like me, and they are hunting for as many models of their home road as possible. I'm actually using focused prototype modeling as a method to manage my budget as well. There aren't all that many models of New Haven prototypes available. There are even fewer that are accurate, and fewer still that fall within my era. Manufacturers (and probably modelers as well) seem to focus heavily on the era just after mine: the McGinnis era. The cars are more colorful, and the big NH is instantly recognizable. But I'm not a big fan of the paint schemes myself.
In any event, it means that I have a relatively short shopping list, and I'll usually only find a couple of things (if any) at a store or a show. Most of them I have to order, and these keeps expensive impulse purchases at a minimum.
Once I identify what's available, I can prioritize. Price plays a part here, but it's usually dictated more by scarcity. I start with the hard-to-find things, because the longer you wait, the harder they are to find. The next priority is the limited releases that are available now, but won't be for long. New Haven lettered locomotives seem to be the biggest culprit here, they tend to last less than a month on the shelves (at least in CT).
I can easily see the next couple of years focused almost exclusively on filling out my New Haven roster.
Foreign Road Phase
Three major things will influence my purchase of foreign equipment. The first is the scarcity issue. This applies primarily to models I'll be making based on a magazine article. If the model is a limited release I may pick it up so I'll have it when I need it. These will probably sit on the shelf while I'm building New Haven models, though.
The second factor that will play a part is the completion of a signicficant amount of the layout. I'm not talking about scenery and structures. This is purely related to trackwork. When there's enough trackwork in place to start some early operating sessions, I'll need some cars. It might be OK for a few sessions to have all New Haven models, but I think that I'll want to see some variety pretty quickly. In fact, this will probably be important once there's a significant amount of trackwork to have a good amount of cars sitting on the layout.
The final factor, and the one that really signals this phase, is the near completion of the New Haven roster. Once the layout reaches a certain point, almost all of the new additions to the roster will be foreign roads. This is due to several reasons. This will happen from time-to-time when there aren't any specific models on the market. But the roster itself is based on the real thing and the size of the layout. So I know exactly what I want for the roster, and once the New Haven portion is complete it's not likely to change much.
These phases aren't set in stone, of course. They will always overlap to some degree. But the major factor for me is that my focus will shift. I might pick up a nice model of a foreign road because of a particular opportunity, but I won't be searching out those opportunities. Right now I'm looking for New Haven stuff, and just New Haven stuff. I already know the next 2-3 cars I'll pick up, and that will probably complete my flat car roster. This is simply because the models are available, and I don't need very many.
How quickly I build the roster also depends on what prototype info I find. If I have a choice between an Employee Timetable or a locomotive, I'll always go for the timetable because they just don't show up very often. Once the actual construction on the layout begins, then funds and time will be focused on that aspect. This, in fact, forms another series of phases: modeling. My situation would also be similar to somebody living in an apartment or going to college. No layout (at least not at home) so plenty of opportunity to plan and build a roster. Once the layout gets under way the roster probably won't grow much until enough of the layout is ready for some trains. Then roster building gets to fight it out with scenery and structure building for a few years. Once that's largely completed it's back to roster building.
This is all assuming that you don't move and have to start with the planning stages again...
Anyway, my current goal is to work on the core of the New Haven roster for the next 6-12 months. I may have less time depending on when we move. But I'll still have a window of time after the move to build some models while I work out the final plans for the layout befure I start building.
This weekend I'll be adding a new locomotive (or two) to the roster. They won't be super-detailed yet, and they won't even be the correct road number. But although they were released a couple of months ago, they are already getting harder to find. By the end of the month I'm hoping to have 2-3 more flat cars to build, and then I'll be looking to get started on the PS-1 boxcars. With a tax return, bonus, and a tax rebate all due in a couple of months I'm hoping to be able to build all 20 at once. With those complete I'll have about 15% of my freight roster done.