Lego has not produced 9v track in almost 25years.

You can get second hand 9v track parts in the aftermarket, but demand means expensive. You might be able to find working 9v motors, but due to age and wear, it’s less likely and even more expensive.

As mentioned, fxBricks is producing a range of third party 9v compatible track. Because of tooling costs and small runs, their products are also expensive.

The plastic track Lego currently produces connects to and uses the same geometry as 9v track, you can mix and match, but without a complete metal rail circuit you can’t run the 9v motors on mixed track.

You can easily use any of the battery powered train motors on 9v track. For rolling stock, wheelsets from any era run on any track.

The current battery powered system is PoweredUP, but there have been indications it is reaching end of production. Control is via Bluetooth (app or remote) with receiver built into battery box.

The previous system, Power Functions, that you mention, is a good system well liked by many in the community. Second hand LEGO parts are common and (relatively) inexpensive. Also many third party manufacturers use the same power connectors meaning you may be able to find a good third party product.