Workflow question

I am about to start modeling a reverse trike (Motorcycle powered, two wheels in front, one rear). Some of the parts of it require a high degree of detail; wheels and tires, chassis, engine, suspension, interior, for instance. Because of the possible disastrous consequences of doing a Select All and then a modification it seems imperative to keep most of the main parts separate until they’re ready to be grouped together in the finished mesh. I say “grouped” because this will be a static model. I’d appreciate hearing the pros and cons of modeling the individual parts each in its own file and then appending them or modeling each on its own layer and then joining them in the topmost layer. I’m leaning toward the layer approach, but I’m open to other suggestions.