When you sign up for a prepaid plan, they will give you a SIM card for free. In regards to the $50/mon. plan, only the first 100MB of your data that you use will be received at 4G speeds. After you have used 100MB, your data speed will be reduced to 2G. Your data will remain unlimited mind you, but it will be EXTREMELY slow. If you plan on using the web for streaming music, video or taking advantage of your phones navigation frequently, then I would recommend a plan with a higher data limit. I am currently on the Tmobile prepaid plan of $70, which gives me the first 5GB at 4G speeds. After that 5GB has been used, the data transfer rate is slower than snail snot.
Since I use my phone as a WiFi hot-spot quite frequently, especially for use with Netflix, I decided to do a test to see how much data I used. So, instantly after renewing my $70/mo. prepaid plan with Tmobile, before I used any data for anything else, I connected my phone to my computer using the WiFi hot-spot feature and started a movie on Netflix. After 30 minutes of the movie playing, I stopped it to check what my data usage was. It was 355MB. So, 1hr. of WiFi hot spot usage at 4g speed=710MB of data. That equates to 11.83MB/min. I added up the total time of 40 movies that I had recently watched on Netflix and came up with an average of 115 minutes per movie, which equates to 1.33GB(rounded up) per movie.
I know that took a long time to explain, but what I'm trying to say is that you might want to think about a different plan that includes more data, even if you aren't planning on using something as data thirsty as Netflix.