Loads of ~£20 mega bright lights on
ebay, they usually come with battery and charger and a couple of different size o-rings to suit various handlebars (my partner recently bought one and it came with a head strap too, handy for off road running etc).
You can easily maximise the lifespan and weatherproofing of the light with a glob of vaseline and some electrical tape. Additionally, a little bit of flexy plastic and some tape or an elastic band can just about solve the issue of blinding motorists.
I've been using one for several years. The only issue you are likely to have on a cheap light is a duff battery since it is battery technology that tends to drive up the costs, not the light source. But you can buy replacements. Probably worth having an extra anyway to use on rotation.