Bandos chestplate and tassets. The prices are varying, but they are the most popular items to buy If you're in the market for RS gold cash. 12M for one item, 12M for the other. They grant extra strength and prayer reward.
I'm not an expert when it comes to the helms but if you're killing you need the complete slayer helm for its bonuses (likewise Salve amulet if it's relevant) as well as Verac's helm.
In terms of boots, you'll prefer dragon. There are more comfortable boots, known as "ragefire" I believe but I'm not sure buy Runescape gold you'd want to invest your bank on these. For gloves, choose barrows gloves from RFD or potentially goliath gloves that are from dominion tower.