Hi Greg,
A rangefinder is needed to use optical flow. Sonars are usually not very accurate and I guess that is the reason a driver for the one present in PX4Flow was never added to ArduPilot. If you just want to use the rangefinder for experimenting with optical flow I guess the Maxbotic one is probably enough - you can always change rangefinder later.
The Pixart one I talked about is supposedly a cheap optical flow sensor that is expected to lower prices in this market (we basically only have the “expensive” PX4Flow). If you want to wait maybe there will be cheaper solutions available, but I have no idea of timelines.