Cheapest (but still useful) Unlocked GSM 4g World Phone

I'm traveling to Spain for a week, and between maps, translation, and transportation (Uber), really don't want to go without a smartphone... but, I'm a Verizon user at home, so I'll need a cheap GSM 4g phone to use with a prepaid SIM from a Spanish carrier.

Any suggestions? I see plenty of BLU phones available for $50, but if they don't have the battery to last a day, or the power to actually use basic apps, they wouldn't make sense. I was hoping to just grab an older, used phone but they aren't terribly cheap either. Maybe there are better places to look for them?