You're going to need to do a little experimentation to find out what works best for you, but here are a few things to consider:
First, the minimum amount of light you need depends on the performance of your camera and lens. Stanley Kubrick shot parts of Barry Lyndon in candle light, but he used a f0.7 Zeiss prime which was built for NASA and customized to his specification.
Second, light falls off as a function of an inverse square. So it really matters how close you can get your light to your talent. A light that's 22 feet from its subject will appear twice as bright when placed 16 feet from the subject. This doubling occurs at distance intervals which you'll be familiar with as a photographer: 22, 16, 11, 8, 5.6, 4, 2.8, 2, 1.4, 1.
Of course, you probably don't want to put a 1K HMI right next to your subject because he or she might melt or catch fire. This brings me to my third point:
Not all lights are created equal. Day by day, more efficient lights are available for film and video. LEDs, fluorescents, and plasma now come in a variety of form factors. These efficient lights are particularly useful because they can be placed close to your talent, without generating too much heat. However, if you choose one of these kinds of lights, be sure to evaluate its color accuracy (color rendition index or CRI). Tungsten lights output full-spectrum, 100 CRI. Cheap fluorescents are notorious for having a greenish tinge. The problem with poor color accuracy is that it's nearly impossible to fix. A simple white balance won't work.
Lastly, since you're talking about shooting interviews, you probably want your key light to be soft, so that it's more flattering. LED panels or Kinos are great soft light sources, but the price you pay is versatility. A hard source can be made soft (through diffusion or reflection), but a soft light can't be made hard.
Personally, if all I were looking for was a key light for a medium to close-up shot in an interview situation, I'd be prepared to spend around $1500 on a 2bank Kino fixture, bulbs, mount, extension cord, ballast, and c-stand. [edit] But now that I re-read your question and see that you're talking about lighting 5-6 people, I'd probably expect to spend closer to $4000. We just spent $6k on lights, giving us the capacity to strike 22 4'bulbs at once. I think this would be slight overkill for your situation.
A more objective answer might be that, according to Wikipedia, 1000 lux is typical for studio lighting. ARRI has a handy calculator to give you an idea of what you need to get your scene to this level.
