As of right now 3/2024 the G7 app requires at least iOS 16.2 and watchOS 9.1 Dexcom G7 on the App Store
Apple says Apple Watch 4 and newer (includes the SE) will run the current version, watchOS 10, Upgrading from 9 to 10 requires at least an iPhone XS or later with iOS 17. I think 9 required at least an iPhone 8 with iOS 16.
@Barb27 Check out the Watches compatible with Dexcom G6/G7 and when is direct to watch coming? topic for more info on options. With regards to buying a Watch SE this year, this is more computers than diabetes, it should work, note that it might reach end of support next year.