Magic or Ranging potions to 84 are probably your best bet as they can be turned into extremes then overloads later on. Magic potins are about 17gp/xp and Ranging potions about 21gp/xp at the moment prices change fairly quickly.
You could probably get an Arcane shield for Glacors for somewhere in the region of 30-40m. Not bad considering they were 55-60m almost 3-4 weeks ago but investing in brews is probably the safer investment.