@Chromate What is the water medium you intend to test and operate in?
If it is freshwater (lake), you can get by with using the off-the-shelf cheap brushless motors as long as you silicone spray the moving parts well. The bearings are usually the first to go. I’ve seen several people replace the metal bearings with ceramic ones and they’ve worked ok-ish.
If you plan to operate in saltwater or chlorinated water, the T-100/200 and M-100/200 series are your best bet for long term use. I remember back when I was building my first ROV frame from scratch for open ocean use and the real breakthrough was the BlueRobotics T-100 thrusters. 3 years later and they are still working great.