Carry on but I give no promises to be able to answer all your questions.
Yes everything both ways including email attachments is included. But I would be very surprised if you get anywhere close to 15GB per month; the most common packages are for just 3GB per month which must mean that is enough for most users, and there are several companies that offer different deals for that amount.
This site
http://www.thinkbroadband.com/mobile-broadband might provide all the info you need.
I have my laptop on many hours every day, doing whatever I like in terms of downloads, watching video streams, playing online games etc. and it just mounts up.