I wouldn't use 50mm screws - the old rule of thumb is the screw should penetrate at least twice the width of whatever you're fixing, so with tp (22mm thick - at least mine was) you need screws around 65mm long.
This is one instance where you should forget magnet mart or bunnings - go to a supplier like
http://elraco.com.au - they'll have the screws on your doorstep in a couple of days. It'll cost a little bit more than the 50mm screws, but it's not much, and if you're going to the trouble to build a deck you may as well do it right.
Oh, by the way, definitely go stainless steel. You can have a good looking result with better components (even though some will think of it as base level).
Cheers,
Adam