Which power adapter for watch Series 7?

Just to add to Deggie's comments... A USB-A/C adaptor won't give you fast charging, it needs the native USB-C output on the brick. If you have a USB-A charger from an older Apple Watch it will still charge the new watch, but at the old slower speed.

There is no advantage to the 30W power brick if you only intend to use it for Apple Watch Series 7, the Apple 20W unit is more than capable. The only reason to go with the 30W just for a Watch is if you are a regular international traveller and you want to use it with the interchangeable duckheads for overseas power sockets.

You have identified the difference when you mention the different voltage ratings and capabilities. Basically the watch charging cable negotiates with the power brick and selects the required voltage/current from the brick's available range. The 9v function is needed for fast charge. Other devices connected by a USB-C cable are slightly different because the same cable could connect any device so the negotiation is done by the device not the cable, and some of the higher rated supplies have additional voltage/current capability for even higher rate charging (which the Watch cable can't use).

You would be hard pressed to buy a 25w Apple adapter but the 20w adapter from Apple, or the the Anker Nano, or Mophie 20w power adapter will be ideal for your Watch.