generally speaking, 3 types of tyres available on market: Pure rubber, Silicone/Rubber and Foam. In terms of grip, foam > silicone/rubber > pure rubber. If you go to MMS, you can only use silicone/rubber or rubber as they don't allow foam. CT allows all 3 types but best use silicone/rubber there. Using foam there will require constant tyre-truing to remove silicone oil picked up after several rounds of play. RCPHK does not allow silicone/rubber as they use RCP track rather than carpet. Foam runs best there.
Racing at a track is a lot more fun than playing at home.
generally speaking, there are 2 types of motors on the market: Brushed and Brushless. I believe yours is NOT brushless. If you're new to RC car racing, I would recommend you to just stick with the stock motor and get experienced before changing to faster motor. This is what most experienced racers will tell you. But if you believe you can handle fast motors right away or you would want to learn the hard way, you may go straight to PN33T. (I went to fast motor right away when I was a newbie because I knew I could handle that and I did not want to waster time on going step-by-step 8-))
PN50T is not the fastest but also pretty fast.
Brushless motors is another story.
Forgot to mention an important point. If you choose to use 33T, you must add FETs or the circuit board will very likely be burnt out. 50T does not require adding FETs. For FETs, see this link (http://www.mini-z-guide.com/mosfet.htm). I use 8858.