Extender Models

You are correct in thinking ahead before purchasing.  From my experience, Furrey and shumako are correct.  An RBK50 (one router and one satellite) provides enormous coverage for the typical house.  Things to consider include:

Where must the router go?  Advice is usually to place the router in the center of the house, but many people have their internet modem located in one corner and no practical way to relocate it.  (Installing ethernet cable through two-story houses is often impractical.)

Home construction plays a big part.  Stone walls, concrete, brick, chimneys, metal ductwork, etc. can block WiFi signals.  You need a reasonably clear path between the router and satellite.  i.e. if you have a nearly dead area (one bar at 5G), the backhaul link is not likely to work well if the satellite is placed in the dead zone.

There is a huge difference between RBK50 and RBK23.  The RBK costs more because it has more and larger antennas and a separate set of antennas for backhaul to the router.  You'll get more coverage and more speed with RBK50 than with RBK23 (if you need more).

What are your expectations for coverage outside?  (enough for email and web surfing or do you want streaming video?)

The BIG reason to purchase a mesh system like the Orbi is to have a "seamless" network where there is only one SSID for the entire house that includes both 2.4G and 5G.  Devices connect to the device which gives the best signal and change automatically when they move around.  "Extenders" are another way to expand coverage, but a lot more complicated to manage and you would not purchase an Orbi with the idea of using WiFi extenders.

Fortunately, you can buy an RBK50 at places with generous return policies (Costco, Amazon).  Put it in, and if it doesn't work you can return it.