Don't convert plat directly to gold. Use your plat to buy ankh kits and elixir kits (damage reduction, damage bonus, luck, xp lixes are always selling well) in Store and sell them in CS. Keep some xp elixir kits that you can sell later when expansion starts. You can then use part of the money to merch, for example by buying some spider eggs now that you can sell later when the prices are better.
For farming planar contents it's better if you belong to a guild that can help you clear the Tombs and show you how to deal with Arena and where there are people still farming those maps. As to gear and build, if you click on my sig, it will lead you to my guild thread. The second post on that thread is a basic guide we made for our guildmates with some gear and pet recommendation for players who want to start farming elite on a budget. I hope that helps. Good luck!
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