Don't let people sell you flashlights unless they're super cheap and super reliable. :) Especially avoid buying rescue or air defense searchlights - if a product contains the word "rescue" or "defense", its price will cause a jumpscare. Optics isn't a secret art, Wikipedia has all the relevant information - and if you happen to have a solar concentrator with a reasonable focal length, you have a searchlight waiting to happen. :)
Photo taken from 968 meters.
P.S. Just remember: LEDs need to be current-limited and cooled. :)
Generally speaking yes, but there are some exceptions.
For example, flashlights intended to be attached to a firearm DO incur a premium for a good reason. They're subjected to intense sudden movement and vibration, repeatedly, and it takes a lot of extra hardening and redundancy to develop a light that can repeatedly withstand that kind of abuse without eventually knocking core components loose. If you were to try to attach a tactical light intended for airsoft/bb use to a real firearm, its not going to last long. There's a reason gun lights are as expensive as they are.