Well other than really early Dura-Ace, Shimano rear derailleurs are not speed specific. There is no such thing as a 7/8/9/10 speed Shimano derailleur. Things get weird with 11 speed and 1x drivetrain and clutch derailleurs though.

My recommendation is Campagnolo. You can build a Record or Chorus 10-speed build so cheap it's absurd. However, I'd recommend 9-speed if you don't need triple/long cage setup. Also most people confuse a Campy 10-speed medium cage rear derailleur as being a long cage, which are "longer" yet. If you need triple/long 9-speed is more expensive than 10. If you need just a double setup 9 is less finicky. If you use Syncro 3 barend shifters, you can swap the index ring from 10 to 9 speed for $10 shipped.