What sidemount harnass and wing you have to buy is difficult to say. I dive the xdeep stealth myself. Is the xdeep the best? no. But for me it works. I have changed a lot on the harnass. As I dive with a 400 grams undersuit and drysuit and ali cylinders I need a lot of weight. But why did I bought the xdeep stealth? Becuase of the wing, it can be taken or not. You don't have to take it off from your harnass, it is just a single piece. When diving in drysuit and some small caves in France I don't use a wing, just the harnass. When I need the wing, I take it.
Try before you buy is the best way. But it is impossible to try all systems. And there is always a seller that has its own preference and will tell you that. If there are shops that want you to try more than 1 system, then try all they have.
As regulator I use a ds4 and xtx50.
Then dive a lot. Best is with good buddies. And then if you think you are ready for cave, talk with an instructor.