@bjacobse With two extruders you load up each with your most common filaments and thus do a filament change less frequent. Then it is a "nice-to-have" but nothing cruicial at least not for me who rarely combine materials in same print.

In more advanced use when people want to print for example support structures in one material and the item itself with another filament this is not practically possible with less than two extruders.

Changing colors can be done with one extruder but frequent changes for same part are of course a pain.

So depending on use the dual (or even more complex) extruders are everything from just nice to a must. For a beginner it is not needed (and for something "home built" it is most likely possible to change to a more complex extruder after some years).

@bjacobse Dual hotends are quite nice to have. I often have ABS going to one of them and PETG to the other. I do not use this to change colors during printing (actually have just tested it a couple of times) but being able to slice for extruder 1 or 2 means that I often can print without changing the plastic because my most common choices are those two qualities.

@skywatch Thank yo so much.

@cyberthom Thank you for the comment. No, I have not published the model anywhere and mainly because I do not have the time to support others trying to build it.

Like with any other big 3d-model like this it is also hard to guarantee that all small changes are incorporated into the model. Also, to build the various parts a fairly good 3D printer is needed (all plastic parts are made of ABS using roughly 100 degC bed heating).

@pihome Sorry I missed the little word "smd". It is of course possible to use a soldering station also but is not always so simple. I have used the butane driven Dremel soldering tool mainly because I have nothing else. It is not easy to control the heat with that one.

When I started with 3D printing quite a few years ago I bought an Ord Bot Hadron frame and then assembled the electronics from bits and pieces bought here and there. The printing table is 200 x 200 mm and the useful height is somewhat less but it has all functions needed. During this assembly process and years thereafter of printing I learned a lot and when the Ord Bot started to feel a bit small I began thinking about something bigger.

So I designed from scratch a much bigger one (printing volume 450 x 450 x 500, footprint about 700 x 700 mm, two extruders and a 220V heated bed) based on the CoreXY principle and this became a really good printer which has been working well for a couple of years now. All plastic parts were printed on the Ord Bot (I attach a screen dump from the 3D design model for info)

So my advise is much along the lines as mentioned by @skywatch - if you have the skills, time and facilities build one yourself. You will never regret it (except for during some memorable events during the process ).

If building one yourself is out of the question do not look for the cheapest options. It will just take a few prints before realizing that a heated bed is needed, that more advanced software is a must, that network connectivity would be nice etc. etc. Well known brands like a Prusa I3 is a good choice but there are cheaper options (clones) out there as well but do not expect getting much or any at all support for the cheaper stuff.

The best one I ever used is a Weller WS81. Heats up in a few seconds and even if the unit itself is a bit expensive the spare soldering tips are dirt cheap (even the originals). With a power of 80W it can be used for quite big stuff too.

@masmat Also I use the "native" OpenHab MQTT binding (my version is 1.11.0) and it works really well with OpenHab 2.2 but , as @bgunnarb says, it is a manual procedure to define the items.

I have not tested the MySensors OpenHab binding because I have a number of sensors of own design which I guess will need quite a bit of redesign to fit into this picture, if at all possible without modification of the binding.

I have not tried the Pi as a gateway so cannot comment on your second question, unfortunately. As the Arduinos and ESP work well as gateways and my Pi already serves OpenHab and Mosquitto I will keep it that way.

As mentioned there is an OpenHab binding you can take a look at.

I started using OpenHab long before this binding was available so my setup is to use the OpenHab MQTT binding.

Technically it is quite simple, you seem to have all what is needed. OpenHab and Mosquitto can run on the RPi and a MySensors MQTT gateway sends the sensor messages to Mosquitto over your network. The Mosquitto passes all messages on to OpenHab.

Now my gateway is based on WiFi using a Wemos Mini or other ESP based board. I have also used an Ethernet MQTT gateway without any problems.

At the OpenHab side you need to create Items which listen to the MQTT messages and then the OpeHab rules can act on these messages and display results using the OpenHab sitemaps.

There is a lot of info available about how to set all this up so Google plus MySensors and OpenHab docs/forums will give you the details needed.

This rough description outlines one "manual" way of connecting to OpenHab. Also the serial gateway can be used but I think MQTT is the best option. I presume that the mentioned binding can simplify things but I have no experience of that approach.

I am using OpenHab through MQTT and it is so that the controller has no builtin functionality to respond to the startup message from the sensor. I had to make an own startup message to be picked up at the conroller and then use a rule to send the old pulsecount back at at startup.

There should be several discussions at the forum about this so the info is here somewhere.