The phone is pretty solid with latest updates. If you have the original firmware, it crashes a lot. If it says Android 2.3.5 you will want to download the latest Kies from samsung.com and get it to update your ROM. Or you could go the custom ROM route... but the updated stock ROM isn't bad. Once custom recovery is installed you could use it to remove some of the bloatware.
Not sure if you will be able to get that $30 plan with a used phone though. As long as you are activating a new SIM online at t-mobile.com, it might work. Good luck.