You could buy the RSGP from Sythe to purchase the materials, make the air battlestaves and then sell them. With the money RSGP you made, sell it on Sythe.
You could use a bot that does the Livid Farm minigame. I have had no experience but as stated in CosRob's thread on a guide to botting, the best bot for farming is the Livid Farm bot.
I don't think it is worth it to get 65 defence because the risk when PKing is a bit high (correct me if I'm wrong) and just stay on 45 defence for Beserker Helm. However, you should not train defence but intstead quest it. The link below should give you an idea on what quests to complete. I...
If you are really rich and still want to make money, you can clean herbs with a bot.
The price of clean herbs are more expensive than the grimy herbs so select one with the highest profit margin. Buy as many as you can so you don't always need to turn the bot on and off to buy more herbs...
BGS is used mostly for PvM as the special attack has the possibility to reduce an opponent's defense. Nonetheless, you could still use it for regular attacks in PvP and deal quite a hefty amour of damage.
This guy is a legitimate vendor. His upgrade service is fast, smooth and cheap. I have purchased many upgrades from him and I will keep purchasing from him in the future.