Hello, Santiago. I've heard good things about Problogger. Other writers I know say they've had luck on Upwork.

I think it's difficult to find good writers anywhere. For me, it takes lots of trial and error, followed by lots of training. I actually think Medium is a brilliant place to find writers because you can already see how well they write.

As for a budget, I'm sure you can get it cheaper, but I've not found very good writers for under $35 per 2,000-word articles.

If you target very long-tail and low competition keywords, you could probably get away with 1,000-word articles. Therefore, I'd say 50 X 20 X 10 = 10,000 if your goal is 500 articles as fast as possible.

Of course, you could scale this budget up or down depending on your goals, needs, and budget.