Before diving into filter recommendations, let me ask two questions:
-Why do you want to go the RO route?
-Is your model directly plumbed?
Water filtration is important for coffee and espresso, but you obviously know that already. Choosing the appropriate filtration solution is determined by a few things. Namely, do you have water impurities impacting flavor, do you have water impurities impacting machine functionality, neither, or both?
In my opinion for espresso machines, the most important thing to consider is water hardness (impacting functionality). If the water is too hard, the inside of the boiler will slowly fill up with scale until it is eventually inoperable. This is a problem. But on the flipside, if the water is too pure, the machine will struggle to function properly, and the espresso will be flat and lackluster.
The probes inside the boiler (specifically the level indicators) need conductivity to work. Pure water (a la distilled water) is actually not conductive. Also, coffee extraction is better with some minerals in the water. Here is an article that explains some of the roles of cations in extraction: http://pubs.acs.org/doi/pdf/10.1021/jf501687c
RO filtration may be detrimental to your cause if flavor is the biggest driving factor. RO removes almost everything leaving a level of minerals low enough that extraction may suffer, but you won’t get scale… Some systems offer mineral reintroduction post filtration so you can get the exact hardness and TDS you want. I would be curious to know if that is the case for the shops in your area.
If you know the hardness of the water in your area, you can make better decisions. Water test kits are cheap ($15 online).
Lastly, I would challenge you to consider your dismissal of commercial filtration if you choose to go the inline route. For less than $150 online, you can get an Everpure ESO7 filter and head. If you are willing to drop several thousand dollars for a home setup, invest a hundred to keep it in top shape!
For whatever it’s worth, we use the ESO filters at my shop. They get us in the 50 ppm hardness range which is the recommendation for my espresso machine from the manufacturer.