Hi,
I know some guys that have done the selection.
You def need to be physically fit.
There is a kit list that includes an AEG.
I'm sure you have seen it.
http://www.stirlingairsoft.com/stirling-selection-11.html
I think the less you need to carry the better.
It is most def hardcore and great challenge!
You'll be proud to have passed it, if you do!
Good luck!
John