@Machineage
With that detailed description and a floor plan it would not be hard to provide proper recommendations.
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Type of Deco units (Deco model).
For Internet speed of 100Mbps there is no need for WiFi6 mesh, which is X20. WiFi5 mesh, such as Deco M9 Plus, could even be the better choice. I own M9 Plus and extremely pleased by their performance. They are fine up to Internet speed of 500Mbps, more than enough for what she has today.
Unfortunately, M9 Plus appear to be End of Sale. If you can manage to get Deco M9 Plus set, go for it.
In case you can no longer buy M9 Plus, my recommendation would be to go with Deco X50 or Deco X55, not X20. X20 is very much budget Deco unit and it might lack on performance and features.
Both X50 and X55 are so similar that if you decided you want them, just check their prices (one could be on sale while other is not) and buy model that costs less.
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Location and number of units.
Obviously, Main Deco will be on the First Floor in the Guest Bedroom, where VM modem is. For Ground Floor, the best place fo Satellite Deco would be in the Living Room, right under Main Deco.
The question how may WiFi mesh units you will need is a challenging one. Usually, one could deploy initial set of WiFi units, check coverage and add more to cover places where coverage is lacking.
Unfortunately, TP-Link does not sell single M9 Plus and X50/X55 units. You can't just buy pair and add one more later.
So, to make an educated guess on the initial number of Deco units, you'll need to answer these two questions:
1 (First Floor). Current Netgear AC1900 router, does it provide satisfactory WiFi coverage over First Floor, from Master Bedroom and all way to Bathroom?
Yes: one unit per floor is enough.
No: If she wants WiFi coverage extend to Bedroom 4 and Bathroom, one more Deco unit will be required on First Floor, and it should be placed somewhere in Bedroom 3.
So, either one or two units on First floor.
2. (Ground floor). If answer for 1st question is No, it means one Deco unit in Living Room may not be able to provide coverage for the whole Ground floor. Does she want WiFi coverage reach up to the end of Ground Floor: to pantry and WC?
No: one unit per floor may be enough.
Yes: If she wants WiFi coverage extend to pantry and WC, one more Deco unit will be required on Ground Floor, and it should be placed somewhere in the Dining Room.
So, either one or two units on Ground floor.
Depending how these questions are answered, you are looking at initial 2-, 3- or 4-unit set. TP-Link sells 2- and 3-unit sets. If you find you need 4-unit set, buy two 2-unit sets. In fact, if you think you'll need 4 units, buy single 2-unit set, deploy it and see if by some luck it is enough.