“Going to” is far more common in vernacular English, and it is sometimes shortened to “gonna” although this is informal. Usually “going to” it’s for something that is already decided whereas “will” is something decided while the person is speaking.

I haven’t really heard this as much as “Thanks in advance for your help,” but it basically is just thanking someone for helping you before they have actually done what would help you. For example, if someone is providing you with a recommendation for a job you could use this before you have actually helped them or you could just say “Thank you” or “thanks.”

It’s usually used when talking about a drug so in that example it would be ibuprofen. It’s a pretty common expression. Pretty much just assume that your body is the “system” being about.