Over the past 12 months I've had roughly 100 flashlights come into my possession. Some were purchased; most were sent by the manufacturers for evaluation purposes. Most of them were then gifted to someone else (once I've tested and reviewed them).
For the purpose of metal detecting, I would think that a high power flashlight is not needed (too much for the task at hand – it would blow out up close objects).
One common means of measuring light output is lumens – basically it is a mathematical equation that divides output over a specified distance to get a number that equates to a multiplication of the amount of light available on a night with a full moon. Roughly speaking, a 1,000 lumen flashlight is 1,000 times brighter than the light (in a specific area, at a specific distance) from a full moon.
Our eyes don't see light on a linear scale though, so a 1,000 lumen flashlight does not appear to be twice as bright (to our eyes) as a 500 lumen flashlight. We can see the difference, but it does not appear to be twice as bright.
Human eyes see red, white, and green light quite well. The advantage of red light is that it doesn't tend to hamper our night vision nearly as much as white light does. We also see better with diffused light.
There are a number of brands that offer flashlights with multiple colors [of light] – typically these models are geared toward hunters and those who fish. These types of flashlights are usually a bit larger than most would find comfortable for everyday pocket carry, but they're still smaller than the old C and D models we all used to have. Most of these types of flashlights will run on rechargeable cells – usually a single 18650 battery. They're also geared more toward throwing ability (as opposed to flood ability).
The smaller EDC types of flashlights tend to be single color (white) only [color of light put out]. They're very easy to carry in a pocket and they often have very impressive outputs. Battery choices range from AAA batteries to 18650 rechargeable batteries – depending upon the brand/model chosen. A number of companies offer models (in this size) that can double as headlights (they come with comfortable headbands that securely hold the flashlight).
Key-chain flashlights are smaller still – typically the size of a tube of lip balm or smaller. They tend to use a single AAA battery (or a smaller rechargeable battery) and most feature a means of attachment to a key ring. Outputs vary – some feature incredible outputs for their size – but the highest mode is typically somewhere around 150 lumens (roughly 7-8 times the light put out by the old C and D models).
For metal detecting purposes, if I wanted a flashlight that attached to the stem of my detector I'd look at an edc sized flashlight and get a bicycle mount to go along with it.
One of the models that doubles as a headlamp is also a good choice. I have several and they work remarkably well. I also like the idea of being able to use the flashlight separately when needed.
My choice would be a small key-chain flashlight – something that I can easily attach to a ring or belt loop for ease of access and security. I'd want something with multiple modes; preferably one of the modes being a moonlight mode (1 lumen or thereabouts) so I'm not blinding myself, or attracting attention I don't want. I'd either get a model that comes with colored diffusers, or make a red diffuser myself, for use in a higher mode. I'd want a model that was more of a flood light than a throw light.
Prices would range from a bit less than $10 to more than $100 – all depending upon which brand/model chosen. Nearly all models would be waterproof (IPX 8 – two meters submerged) and impact resistant to 1.5 meters or more.
I seldom metal detect at night, but I always have at least two flashlights on my person (2 on my key-chain and usually another edc model). Most people would be surprised at how often a flashlight is needed on a daily basis.