If you're unable to OSRS gold find a royal crossbow, have an idea of getting the Armadyl crossbow. Always make use of dual weaponry for slayer tasks. Go with the highest tier weapon you own. If you got a lv 80 ranged wep and melee weapon lvl. 80 as your top tier weapons pick the weapon the monster is weakest (according to the combat triangle).
It is unlikely that you will require anything more than bandos/ barrows to slay tasks. If you're going to need weapon that is lvl80 I'd recommend to invest in chaotic weapons and then spend the money on other items. I would suggest that players only consider the lv90 things if they've got money to burn. Up to you though.
For bolts, you may now apply any bolts to any bow. The compatibility aspect has been removed. If you are using level 80 royal bolts in conjunction with an Lv 70 crossbow, those bolts will be capped with a damage cap , as if they were levels 70 bolts. If you have an offhand bow that is level 60 the mainhand bow will be limited to lv 70 bolt damage and the offhand bow will be capped at level 60 damage. Anyway, I purchase Royals since they're the most affordable so it's really not an issue either way.
Ganodermics do pretty well with all things magical. The beta version I tried, when you die, they won't really drop charge either, that's a bonus. Beware of weapons that were previously equipped with special attacks since those special attacks are returning - Check out the combat beta page for rs07 fire cape service more details.