I'm saving up my coins to buy a set of 8 lucky eggs to help me get from 26-30, which is when I'll run out of free ones. After 30 I won't care about leveling up as it just gets ridiculous. I'm currently 24.
I did spend some money increasing my bag storage to 600 however, which I upped to 650 from my first gym coins. I needed the space to store lots of balls and potions. My Pokemon storage is 300 as well, means I have to faff about with transfers less often as I'm usually at about 150-200 that I want to keep either for evolve EXP, duplicates of rares I'm keeping for trading or duplicates that are among my best Pokemon. Like 4 of my best 6 CP Pokemon are Hypnos due to how common they are.