My experience on the ramp was a number of years ago, but I still know plenty of people who work on the ramp.
The way things are done these days, just assume that you have to pay for everything. This seems to be the way of the future. The medical is not an aviation medical as such, but to see if you have any pre existing injuries which may be exasperated by working in ground services, especially back, shoulders and knees. It is also to get a base line of what your hearing is like as it is a workplace with a lot of noise, even with hearing protection. Basically the medical is protecting QF from claims down the track.
In regards to the ASIC, ask someone when you have a chance as I don't know if it has to be issued by QF or not.
As to only being 17, I'm not sure. A lot of ground services jobs require you to drive. So depending on what age you can gain a licence 17 may be ok. Also sometimes they specify about being able to drive a manual.
As to working for QGS, I wouldn't do it. The main reason is that they are very demanding of your availability and the hours don't really suit trying to fit another job or study in around them. Also money... From what I have been told it is a flat rate, so no penalty rates for late nights, weekends etc.(check the TWU award about this). Then there is the management, where the attitude is if you don't like it you're welcome to leave. Not one of these things on there own may make you not like the place, but at some time it will be a combination of these factors. Where there never used to be much of a turnover in this role, there is now.
Having said all that, working on the ramp was one of the best jobs I had. At the time I was there there was a great group of guys that you were working with and there wasn't too many dull moments. I moved on because I knew it wasn't going to be a job that I could do until retirement, because eventually you will get some sort of injury.
Good luck with the job if it appeals to you, but like a lot of things it's not like it used to be. But if you're aiming to be a pilot it does give you a great understanding of what goes into each flights arrival and departure.