A raspberry pi is the wrong choice for this project from my perspective, complexity is far too high for a simple project. I looked at building the same thing (minus watering) because it seemed commercial products were way too expensive (Parrot Flower Power is $60!).

The cheapest DIY solution I could think of was ESP8266 ($2), Vreg ($.5), moisture sensor ($.5) and LiPo battery (i have many of these..) but I decided I didn't have time or inclination to write the software.

I continued looking for commercial products, and ordered one of these: https://www.aliexpress.com/item/Chinese-Version-Original-Xia...

Pros- Cheap ($15). Has temperature, light and 'fertility' (capacitance?) sensor. Cons- Logs to phone app (in chinese) via BTLE instead of WiFi.

After a few weeks it seems to be working satisfactorily and I will probably order a few more units.