The Mixtrack Pro is a good controller and should suffice for a bit longer, until you have enough knowledge about workflow to figure out what the proper upgrade for you should be.
Even is money is not an issue, setting a budget helps narrow your choices down. While the SZ is a nice controller, it is also the most expensive one around and frankly I think it’s way overpriced. The SX2 for example would be as good for 95% of the DJs. Only if you need/want some of the specific features of the SZ (which you will know once you have enough insight into your workflow) would the extra money be worth it. Another powerhouse (also suited for Serato) controller to upgrade to would be the new Denon MCX8000. So there are plenty of options.
The NS7 is just about the only serious option if you want spinning vinyl platters. Again, don’t buy a controller like this unless you are certain that this is a feature that YOU want/need.
Not sure if you have already, but at this point I would say to have a look at the How To Digital DJ Fast course, it will help you check your progress and accelerate it and have you practicing the right things the right way. Also the Pro Mixtape Formula course would help you a lot I think. http://www.digitaldjtips.com/how-to-dj-training-courses/pro-mixtape-formula/
Hope that helps some.