Hey Maxim,
First of all, cool that you are getting into the Steadicam scene! Wish you all the best.
I´ll try and give you my best two cents on the things you brought up.
Going with an M2 because it´s the only Rig you ever tried is maybe not the best idea. Not to say that the M2 is a bad Rig, I own one myself but if you want to invest in something that expensive i personally would advise you to try all the quality options avalible to you (GPI, ARRI, Betz etc.). Now keep in mind that i do not know what brands are avalible to you in china for purchase and servicing. But don´t limit yourself to one Brand and maybe later regret it because some feature or detail in another Rig is more to your liking.
For Arm choice my advice would be the same as above. Try all the Arms avalible to you and see which one you like best. As to the differences i can only say something about the XArm. The Xarm has no Springs like most other arms, instead it has rubber cores (one for each arm sgment) that can be exchanged based on the weight of the setup you want to fly(the same goes for the PRO and Smartsystem arms). The rubber cores give you a noiseless boom from top to bottom. A plus would be that for the price it comes with tools, different sized arm posts and a peli case with custom foam. The G70x I own in comparison comes with a soft bag and arm posts are sold seperately. But its toolless which i like. So it´s very personal to you and your liking.
For switching from Exovest to XSpine i can also say something. I currently own an Exovest and after trying on the Xspine a few times i fell in love with it. As i am a bigger guy it feels better on me than the Exovest. I like that all the small adjustments for size on the Xspine can be done with a ratchet mechanism, so you can quickly go bigger or smaller around your waist, for example after you´ve overdone it at lunch or if you have to put on warm clothing for an exterior in the winter. Also because of the ratchets and srewed in size adjustments it´s just one rigid unit which i personally love for wearing it and also carrying it around. Plus it has a front and backmount so you always have both options. Also you can mount an easyrig to it, which can be nice, if you ike easyrigs. But again, try all options avalible to you and then make your decision. Many people swear by the PRO vests for example or love the Walter Klassen vest.
Long story short, don´t limit yourself. Try everything you can get your hands on, talk to other operators about thier experience with gear, sleep on it and then make your decision.
i hope this was helpfull in some way.
Cheers
Sev