I have about 67m to buy gear for Armadyl. Would I be better off buying full Armadyl instead of a spectral, or should I do a mix of spectral with armadyl plate or something? I'm 99 range 99 defence 99 mage 99 hp 78 summoning and 87 herblore.

Just use a dragonfire shield. It's almost as good and a lot cheaper.

Use void + deflector and an arma body, and a ccbow of course.

 

Plus, you need a uni to kill arma effectively, so I'd recommend putting it on hold until you get 88 summ.

I should add that I won't be soloing

You definitely need a Uni to kill Arma effectively that is good advice so before spending that 67m on gear spend it on gathering charms for a unicorn and seconds. A spectral is a great upgrade over a DFS but personally I would buy full Arma first and a spectral later.

Ya what cheat said, except getting a deflector and using void + arma top is going to do a lot more for you than full arma.