Don't just look at the gross rental yield, because you're going to be disappointed. I know a colleague who bought something similar in Malasaña a few years ago, paid 260k and between building fees (120 euros/month), property tax, maintenance and tenant insurance easily came to 250-300 euros a month, plus unexpected stuff like leaks or pipe replacements that popped up out of nowhere. With 850 euros in monthly rent he was left with just over 500 in actual money after expenses, which is a return of 2-2.5% a year if there weren't months without tenants or payment issues.
What does make sense is if you see the flat as a bet on the area appreciating long-term (10+ years) and not as a source of cash flow, or if you have the money to ride out dead periods without it killing your cash. Otherwise, that down payment gives you more room with index funds or even other types of real estate investment with a better ratio.