Basic Smart House System
Big speeds are not needed in principle, because the data will be mostly the size of the White Two. In a binary form, it'll be with addresses and a check amount of about 5-10 byte. I mean, an estimated volume with a reserve of 20 (10 - request, 10-response) bytes for the device.
In favour of modbus, there is still a huge choice of devices already available. If it might be interesting to smell, to collect something, then in a year or two, something might change and the possibility of buying something might be very good.
I wouldn't put great hopes on the Ardurin, at least as a head restraint. It's better to take either PLC or micro-PC (e.g. Raspberry) with an RS-485 interface. Besides, you should think about building a network. In my view, it's better to have smart final devices with a master's connection. This will help in emergency situations, for example, the light will be operated by the switch on the wall through the device until the master is available.
Of course it's not a short story, there are some underwater stones. For example, it is highly desirable to think about galvanic tying and proper nutrition. But in general, RS-485 and Modbus, there's a lot of information online and if the target is to be dealt with pretty well.