What happens after you use your 18gb for the month? How is your ability to stream videos/movies? I have Liberty 30. I use my 30 in a week or less and then my restrictions start. After that, I can only stream movies in the middle of the night or during the day - and the quality is never good. During prime time when most people would want to watch movies, I cannot do anything related to streaming.
I use a Chromecast and the other members have pretty much nailed it. Chromecast should work fine, but you will want to keep an eye on your data usage if you're streaming a lot of video, especially in HD.
It's also worth noting that the Chromecast may sometimes download software updates or wallpaper images without asking. I don't believe these are huge, but it's something to be aware of. My advice would be to unplug the Chromecast when you aren't using it if you have concerns.
If you use the Chromecast simply to push movies or music from your computer's local hard drive to a TV, that shouldn't use data.
Hope that helps,