The problem with "solo" 401(k)s is that they are sold as simple products, but as rcline notes, they are in fact a full-blown qualified plan. "Solo" is a marketing term, not a definition.
We handle some "one-man" plans (right there, things get complicated because these plans can cover more than one person) and we apply our regular checklist. Some things matter, some don't, and the fact that a 5500 is not required is actually a somewhat trivial relief, because by the time we get done with our testing and compliance work, the 5500 for most plans, at least SFs, is almost an afterthought; preparing it or not is a matter of a few minutes' time.
What I'm saying, and I think what the other posters have implied, is that there is no minimum/easy threshold or shortcut for providing services for just these plans. You really need to know at least something about controlled groups, eligibility requirements, and any and all aspects of retirement plan testing and compliance. Yes they are easier than plans with other participants but without having such knowledge and experience and systems in place I think you are asking for trouble. (For starters, a 5500 is required in the final year of such a plan; who is going to prepare it?)