Camera. Although this may seem like a no-brainer, there are "requirements" specific to this type of videography that will make your job easier if you know about them ahead of time. Many stage productions use continuously variable lighting to create mood, add to visuals, and make your job difficult by requiring you to constantly re-adjust your iris settings. Many dispute this practice, but logic tells you that, no matter what the director or lighting designer have in mind, if you can't see the performance or the performers, the video is worthless. Just keep your adjustments subtle, no more than necessary to create a watchable picture.
So a camera with an adjustable iris is mandatory. Unfortunately, most of the newer digital cameras have a digital iris, which means the adjust ring is electronic rather than mechanical and will change the iris in noticeable steps.
As in wedding videography, a 3-chip camera will provide you with a visual edge. Because of the color processing of a 3-chipper, color bleed or smear will be minimized and give you a much cleaner picture. Also, your camera should have XLR audio inputs. If not, there are adapters available to convert your 1/8" microphone jack to real audio inputs. Two independent channels are mandatory.
Tripod. Again, a given, but be sure to get one with some heft to it; smooth action is key. Expect to pay $500 or more for a passable model. I have also found that a dual-handle unit will give you additional control of pans and tilts, plus it gives your left hand something to do.
Monitor. Even though you may have a newer model with a flip-out LCD screen, you might want to look into using a 7"-10" monitor. LCD monitors are nice (and expensive, for a good one), but I find that CRT models have a picture better suited for the extremes of theatrical lighting. A larger screen is also easier on the eyes on those longer productions. I generally position the monitor just below my line of sight so I can keep the monitor and entire stage within view with minimal head movement.
Audio mixer. Here you have about a hundred choices, and which way you go is a matter of personal preference and size of your wallet. Favorites here are brands like Mackie, Behringer, Alesis, Yamaha, and others. You should shoot for a minimum of 4-6 XLR microphone inputs and at least one line-level input, and tone adjustments of at least two bands (three is better, and gives you more flexibility). If you intend to used an external sound modifier like a compressor, the XLR channels should have inserts. The more features you have, the more flexible you can be.
Microphones. Cardioid condenser mics are mandatory for stage productions; they provide good pickup and off-axis rejection and great tonal quality. How many? It depends on what you intend to cover; 2-4 is a good start. Price here is anywhere from just under $100 to several hundred dollars. PZM or boundary mics are good for some specialized uses and would be considered not mandatory. You should also have at least one good dynamic mic (e.g., Shure SM-58, Samson S-11—$50-$150), used for interviews in high noise areas and mic'ing amplified speakers, and a shotgun mic (Sennheiser ME66, Audio Technica 835, etc.—$250 and up). There are many other variations of these basic instruments that would be nice to add to your toolbox, but they are not needed for basic productions. (Check out recent installments of Lee Rickwood's monthly GEAR & NOW column for more on these last three categories.)
Compressor/limiter. These are a must if you're using a digital camera. Theatrical audio dynamics are sometimes extreme and always unpredictable. A compressor could be a lifesaver and reduce the number of headaches per show by limiting the peaks of "hot" audio to prevent clipping and distortion. My favorite is the Behringer MDX4400 or MDX4600 ($110-$160); both are 4-channel compressors in a single rack space.
Miscellaneous. You'll also need a set of headphones (full ear cup with good sound isolation); mic cables (various lengths, up to 100 feet); mic stands (at least three or four of the tripod type with booms); clamp-type mic clips also come in handy. Other must-haves include various audio adapters (XLR, RCA, 1/4" phone); extension cords (you can never have too many); Gaffers tape (again never too much); and power strips with surge protection and filtering.
If you intend to offer stage productions as a sideline or make these types of events the focus of your business, you will probably have to invest in additional equipment—things like audio snakes, backstage monitors, equipment stands, hand trucks, and ultimately, a bigger vehicle or trailer.
It's worth noting that the above information is aimed at a single-camera shoot and designed for the newcomer to this area of video production. Multicamera and live-switched events require additional, specialized equipment and direction and should not be attempted without experience in producing stage events.