There are plenty of camera apps in the app store that will let you lock of the exposure and focus of the iPad's camera. These are the features that I'd consider most important. Locking focus and exposure keeps the camera's settings from "breathing". This forces you to think about focus and exposure before you start shooting. This forethought is the single most important ingredient for producing quality video.
Improving the quality of your lighting will probably give you the highest Return on Investment. Again, good lighting depends on pre-planning and forethought, but no one can tell you which kit is going to give you the magic results, especially without details about what you're shooting. If you plan to shoot outdoors, you're going to need an entirely different setup than what you'd use indoors. For outdoor lighting, look into scrims, diffusers, flags, bounce cards, and reflectors. If you're shooting ENG style, you'd probably want a light attached to the camera. If you're doing indoor interviews, you'll probably want to research "three point lighting". Whatever the case, it's likely that a nice soft light source should be one of your early investments.
Sound is the next area you can improve to make better quality video. The first item on the sound chain is a microphone, and -- like choosing a light -- choosing the right mic for the job depends entirely on the job. Professional recording environments usually have a huge selection of microphones just to get the right sound. You'll probably want to invest in a lavalier or shotgun microphone early on. Once you have room for more kit, consider mixers, preamps, and recorders.
Camera stabilization is also very important for professional quality video. Again, a little pre-planning will go a long way here. Is your shot going to be static? Propping your iPad on something stationary will eliminate the hand-held look. Are you going to pan? You're probably going to need a tripod or monopod for that?. Is the camera going to move? You're going to want to increase the camera's rotational inertia somehow. This can be as complicated and expensive as buying a stedicam or gimbal rig, or as simple as duct-taping your iPad to something more massive, like a heavy dictionary.
Anyway, the best thing you can do is to research yourself into these areas and find which solutions work best for your situation.
Oh, but here's a gadget that you might find useful. padcaster