RE: Used Gear. I went that route and was able to get a full setup (short of tanks and weights) for what I would have paid for a top end, cold water capable reg set. Furthermore, I ended up with the same or better quality in amazing condition and feel much better (conscience and wallet)... Apeks Tungsten with the 40 octo, Zeagle Ranger BCD, Sherwood Wisdom II Computer, Akona Bag, Atomic ARC mask, Mares Raptor (only regret... damn split fins), high end light plus a few other things. I got some off of the marketplace here and some off of Craigslist... would definitely recommend used.
For the 5 Kg limit, there is no way that you will be doing a full set as carry on. Get a hard case for your mask and throw it below (packed well) along with fins, BCD, wetsuit (rolled to avoid wrinkles) and basically everything but your regs and/or camera if you carry one. Not that the other stuff isn't valuable, but it is a) more durable and b) less likely to kill you if there is a malfunction (exception being BCD, but what will really happen to a BCD down there?!?!). Do a full inspection after landing and you will be golden!