The ion engines used in existing spacecraft produces similar thrust to a piece of printer paper in your hand - the thrusts of the engines are measured in terms

The ion engines used in existing spacecraft produces similar thrust to a piece of printer paper in your hand - the thrusts of the engines are measured in terms of milliNewtons. They are useful in deep space situations, where (due to nuclear reactors or abundant solar cells) you're power rich but propellant poor, and don't have to deal with aerodynamic drag. Over a long period of time, an ion engine can yield big velocities. With drag forces though, they'd simply have no effect on a floating balloon. A balloon could present a method of getting a lightweight rocket to a very high altitude before engine firing, reducing the drag on the rocket and allowing the rocket engine to be designed just for the very low air pressure environment at those heights (rather than the compromise that modern ground-launched rockets have to make), but the size of the payload would be limited by the balloon size (with approximately 1kg/m^3 of helium, you'd need a lot of helium to lift a rocket of any size).