As a private Investigator there is a number of essential items of equipment you require. It is very important that this equipment is checked regularly for serviceability and that you don't get court short. Imagine you're on a job and your camera battery goes dead and at that moment your subject does what they are not supposed to "the money shot has been missed", how would you explain this to your client. A report is nice but a professional video says it all.
Equipment:
A stills camera- Good quality digital stills cameras are available on the market today and you can pick up a good quality lens quite cheap for those close shots. (Buy the spare battery)
Camcorder with spare batteries, for those quality video experiences every subjects hates to see being played back to them. Remember get one with a View finder and night shot for those times when the LCD screen may compromise your position.
Mobile phone-speaks for it self.
Quality radios for communication when carrying out operations as a team. Motorola are always good.
Binoculars for surveillance operations-essential.
Touch- sounds basic but a most at night.
A note pad and pen/pencil, all notes should be made at the earliest moment closest to the event or actions by your subject.
Dictaphone- always good when on a fast mobile surveillance.
A covert camera- many on the market but always look out for video quality and how it performs in the likes of dark pubs etc.
Last but by all means not least is a reliable vehicle- spend your money and get something that is going to get you where you need to be and back.
As a gadget man I have spent thousands of pounds on equipment and will continue to do so, but the equipment above is enough to get started as a Private Investigator.