I have to agree with BaitBucket, it all depends. One point, the Navy uses obsolete rebreathers. For small passages, the KISS rebreathers are good. I use a SideWinder. For drysuits, you want one that is sturdy, because caves are rough on the material. I have an SFtech, It has Kevlar reinforcement. As for undergarments, it depends on what temperature water you will be diving in. I use Weezle Extreme Plus in cold water, and Weezle Compact in Florida. I use a 7mm wetsuit in sumps, since walking in a drysuit gets really hot. You want reasonably stiff fins. In backmount, you want heavy fins, to counteract the weight of the tanks on your back. In sidemount, you want lighter fins , or you will be tail heavy. You want a low volume mask, so if you flood it, it is easier to clear. I use Shearwater computers, with Fischer ports to connect to monitor my rebreather. We use hand signals, and wet notes for communication. Are you British? We call "torches" lights in the US. I use BigBlue dive lights, mostly the TL4800P. For instructors, you first need to decide which instructor. Some specialize in backmount, some in sidemount. For a list of NSS-CDS instructors, see here:
https://nsscds.org/instructor-listing/
Any opinions are personal.