Watch high legend players. Doesn't matter who, as long as they are top 500. You will see what the best decks are, how they think and how they play. Use ViciousSyndicate, read the last meta report but don't use HsReplay unless you have premium - the bronze stats are useless. Finally, sell all your wild stuff, very few cards are viable in wild anyway.
Pick the free deck that offers you the most dust and wait for a few days after the nerfs to see what's good. I'd also spend on the cheapest bundles. Generally speaking, any bundle that gives you more packs than dollars/euros you spend is good.
I think you can have 2-3 top tier decks without even spending 100, but if you want variety it's best to spend it I guess. Also make sure you work your way up to 2000 gold for the mini set coming out next month. Good luck and welcome back, I think this meta is great.