A bit of general information: I've heard informed that Dragonrider costs half the ge price... however, I would go for Bandos regardless. It's super cheap right now. (~6m per Chestplate as well as Tassets) Try to RuneScape Gold build an Slayer helm, and try upgrading it to the t70 version using your slayer points.
In order to answer your question regarding stab/crush/slash weapons, let me explain how weaknesses of monsters work using an illustration. Steel dragons are invulnerable to Water spells. What this means is that you'll get the greatest accuracy when it comes to Magic (Water spells in particular.)
Also, if a monster has a weak ability to Crush or crush, such as Jadinkos Crush, that's a sign that melee is the fastest attack style against them, that is, Crush is the most effective style. In contrast, Range or Mage will have less accuracy. (Although at high levels, your gear is usually good enough that you won't notice a significant distinction.)
For the end, keep your Dragon scimitars and don't purchase additional melee guns. They'll be sufficient for anything that's not suited to melee. Likewise, stick to dual wielding range and magical. It's likely that you don't possess anti fire potions, so avoid dual handling. Also, dual wielding allows the use of a shield and the Rejuvination/Resonance abilities.
Like with elemental spells. Personally, I've got tons of elemental runes that I have stashed away, so I'm always able to RS3 Gold use the monster's particular weakness. It's entirely up to you as you'll have enough success using air spells.