If you’re getting a fiber plan, you don’t need a cable modem at all. Save that money. You just need a router of some sort.

1500 sq ft can usually be covered with just one router usually, depending on the layout of your home. If your townhouse is tall and thin, then where your ISP connection is located will determine if you can just use one router, or if you need more coverage.

Before committing to any system, I’d look at what kind of wiring is already included in the house. Often, coaxial cable (using MoCA Adapters) or old telephone wiring can be converted to Ethernet. If this is possible, it not only reduces the need for WiFi (perhaps you can cover everything with one router and 2.4GHz WiFi in the extreme corners because everything important and demanding is now wired in, so that’s good enough), but also opens your choices up tremendously on what system to buy into not just Orbi.

If you or your partner already have a router from your past home, consider using it for the first month until you evaluate your wiring situation correctly, and then make a hardware choice.

You should look at the postings on this sub. Many (especially ones with pictures) are all about converting coaxial cable into Ethernet, or rewiring old phone lines into Ethernet. Read some of those and you’ll start to see a lot of possibilities for your home network.