I think dbslayer would be perfect for this. Mysql is super easy to set up on a vps (there are lots of scripts to set up the whole server on sites like lowendbox or vpsbible) and then with dbslayer you get json and clustering if you want it so you can scale smoothly. It works great with node and there's already a module for it on Github @:

https://github.com/Guille/node.dbslayer.js/