Well, there's all sorts of things people put into their kids, but I'd suggest a basic kit be something like:
Voice-activated Microcassette Recorder - great for EVP
Infrared Night Vision Camcorder - a real infared camera is hard to find and they're pretty expensive
Temperature gun - easier to measure temperature changes at a distance
Digital camera - or any standard digital camera that most of us have now anyway
That, of course, is extremely basic.
I once read a guy claiming that digital stuff was junk and researchers should only use film cameras. I don't remember why something about the actual spirits being captured witihin the film. Any thoughts on that? I can't try because it's so much more expensive to do film than digital. I don't think I even have a film camera.
@lily, that seems more superstitious than anything, I can't really understand any difference. I guess digital cameras decrease the chances of double exposure, so they don't like those odds, plus also digital is less likely to create bizarre artifacts.
i personally have experienced different stuff with real cameras, i think digital ones might just be bad because of the digital processing, i'm not sure.
You can start slow and inexpensive. Probably two must have is a digital recorder and an EMF meter. The lower cost units are OK to start with as it will be helpful when doing a walk through for baseline readings. Here you will find where heavy amount of electric current are in walls, floors and ceilings so you can write down these areas. When your investigating you don't want to think there is an occurance because you forgot there was an electrical panel next to you.
We've tested Mel Meters, Trifields, 822-A, Ghost Meter and Gaussmeter and around AC current in the 50 to 60 Hz they all do react. Feeling watched or uneasy is one of the main complaints from most people who feel they are haunted. This is why the EMF meter is helpful. The more expensive meters do lower range of Hz and some also can read DC
If you get a spike in a EMF meter that will be a good time to take out the digital recorder for an EVP session and take pictures if you do have a camera. The Vivitar DVR 510N shoots in 0 Lux. Go on YouTube and check out some reviews. You can do video and stills and a used one is pretty cheap.