Which is better - a modular boiler room or a special room?

Good day, Sergey. Both options are possible. Depends only on the budget and your desire.

Three negative factors of a modular boiler room:

1. Accessibility to strangers. A powerful argument in your case.
2. Solid fuel boilers have a more complex system compared to gas ones. It is easier to arrange efficient heating in the building. In Figure 1, the simplest diagram is shown. It is possible to achieve the promised boiler efficiency according to the passport, at least using a buffer tank. Such a scheme leads to fuel savings of up to 20-30%. An example is shown in Figure 2.
3. Manufacturers recommend the temperature of the return coolant in the boiler at least 60 degrees. This is due to the formation of condensate, with a lower rate, its formation is inevitable. In turn, this affects the life of the boiler. To implement this recommendation in a model boiler room is quite difficult.

In general, the capital structure will always outperform the metal modular one. You did not indicate the area of ​​the cottage, so purely theoretically I can say that a boiler room of 6 m2 is suitable. This area is quite enough to accommodate all nodes and solve the issue of hot water supply. Installation of indirect heating in the boiler system will allow you to get hot water in the heating season without additional finance.

An interesting scheme would be combined heating with a solid fuel boiler with electric. It is also possible to link an indirect boiler into such a scheme. Figure 3 shows a diagram. And I recommend that you familiarize yourself with this article - It presents the proven manufacturers of solid fuel boilers.