I don't think we (us modelers) ever talk about the real 'stress' that accompanies a layout project. I guess if we admit that its in some way 'stressful' we open ourselves up to even more question from folks who think we do this for leisure! But its true....there are so many options and so many things that each of us want to accomplish with our layouts, that is actually a very difficult process of eliminating choices and committing to a direction (and this commitment will come at a real cost in both time and money!).
I'm not sure "stress" is quite the right word, although that may depend on the individual. Certainly the process or deciding what you want to model, what you can afford to model, what you can fit in available space, and how to bring that to reality with whatever skills you have, is for most a long one, and potentially stressful. On the other hand, some people enjoy that part of the process as much as (or more than) the actual construction. I've found the planning and construction of my layout to be the opposite of stress, as it's something I do after work to unwind, and it's been very good for that. But I do have the advantage that this is my second layout, third if you count my initial table-top temporary Japanese layout that followed the old HO U.S. freight layout, so I have a realistic appraisal of my own limits.
There is the potential for stress in any hobby, and in all aspects of model railroading, not just the design aspect. If you expect too much, and take the inability to achieve that as a failure, rather than as a temporary setback, you will certainly create stress for yourself.
I spent about three months planning my layout (after nearly a year of thinking vaguely about what I wanted while continuing to expand my original). And during the year (so far) of construction, design re-visits have been ongoing as I fine-tune things. I don't recall that as being stressful, although it was at times frustrating when something I wanted just wouldn't fit, or when things didn't quite work out as expected. And the shortfall between what I can see in my head, and the reality of what I can create could be a source of stress if I were trying to look as good as one of those magazine layouts someone spent five, ten or twenty years building. Maybe someday my layout will be one of those, but that's not what I'm trying for in "version 1".
I think the key to avoiding stress lies in a) not taking this too seriously (layout building is, after all, a hobby for most of us), and b) having a realistic view of what you can accomplish and starting with something comfortably inside those limits, then growing to (and beyond) them over time. In particular, I'd suggest a new builder get trains running as soon as they can, even with partial/unfinished scenery or electronics. The satisfaction of building something that works will not only be a reward in itself, but it will provide motivation to do more.
It's easy to take on too much, particularly if you view a layout as "the" layout, and not just the first of several you will ultimately build if you continue in the hobby. And trying to do too much would be stressful. Layout building is about trying new things, and learning new skills, some of which won't work, or won't work the first time. And design is just another part of that. No design can ever be perfect, and any design will be revised over time (or perhaps ultimately discarded and replaced with something else).
In a way, I think modeling magazines do the hobby a disservice by promoting the illusion that all layouts look as good, and are as well designed around some well-researched theme, as the ones that are good enough to be published. I enjoy those articles, and I aspire to someday being able to create something like that. But I think a lot of would-be modelers get discouraged when their skills aren't up to that kind of vision (I know I was, for many years). Modeling skills aren't something you're born with (although some perhaps have more talent for certain skills than others). Skills are something you learn by using them, and by making mistakes. And that guarantees that what you can do on your first layout won't be as good as what you'll be doing a decade later, if you remain interested in the hobby.
