I have one of the bontraeger “Ion Pro RT” models (pretty sure that is the one), and it is very bright, I have done lots of descending at night using this and it is quite bright. That said, I don’t really like the charge indicators on it (ie they have the charge indicator go out when fully charged!!! terrible if you are charging it from a USB power bank that might also go empty. The ‘fully charged’ and ‘charge power source not working’ look exactly the same.)
I also have a couple of $30 ‘filixar’ lights with claimed 800 lumens that do very well also.
I think 800ish lumens would be good enough, and the low 1000s would be more than enough. All you are doing by going brighter is shortening battery life. Getting a light with several brightness modes will make it more useful in other settings.
Depending on the length of the descent, and how sure you are it will be fully charged at the top, I would consider having some kind of redundancy for front lights. If you are only going to have a single light, go for a more reputable brand For my descent while everesting I always had two lights. Never had a failure, but one going out (due to failure or just battery dead) during the descent would be very bad.
On the MTB, MagicShine 8000s. “8000 lumens” was measured at ~7800 lumens. It’s night to day where there is absolutely no light at all.
Get a directional (non-flood) on your helmet, so you can turn your head and have illumination where you are looking.
Varia on the rear is decent for the radar, but up the lumens with another light.
You can see some shots on the MagicShine at the end of this video. I will note, it is blinding, which may not be great on the road when there are cars (it’s like a high beam). If it’s a high traffic area, would look at something else, but if you’re doing “illicit stuff” light em up and let it rip at full speed.
I ride early morning most days and use 2 Nightriders, one on my bars, and the other on my head. I have the 650 size now, and running both on low is plenty for normal riding. There is plenty of light with both on medium for 50+ mph descents, and they will run about 2.5 hours on that setting when new. I’ve been using them for 4 years, and totally recommend.
My recommendation is above your budget but is very much worth it at $180. I’ve used a bunch of various lights over the years and always ran 2 lights. I now run only the Outbound Lighting road light, it blows away every light I’ve ever had for useable light and would be a good match for a fast road descent https://www.outboundlighting.com/product/focal-series-road-edition/
I’m sure some of the expensive lights are very nice - even the bontraeger I suggested is over your budget, but seem overkill if this is likely to be a one-time or very limited use item for you.
Here is a nightrider 900 lumen for $50. (I consider this a reputable brand) I have an older one and it has been solid.
Here’s a 1000 lumen off-brand for $15. This is very similar to the two generic lights I bought and used - they both worked fine and I am very happy with them.
I would trust the output and runtime claims from nightrider more than I would from the various off-brands, but my experience with buying ‘cheap’ stuff like this is with a little care in shopping you get a great deal on a functional item, usually with a few rough edges.
You could get both the above for under your budget, and have some redundancy. Another good way to use two lights is to have one mounted on your helmet.
I’ve enjoyed my riding at night - it is a very different experience. I hope your ride goes well.
One is to make sure the battery life of the lights in which ever brightness mode you plan to use will last the length of time you need. Some lights can actually be charged externally while running. The helmet light I use while mountain biking charges with a mini USB cable and will run with an external battery running, adds to battery life as I run the cord to my jersey pocket to a battery.
Two is if you are not comfortable descending and/or night riding, slow down. Don’t out ride your headlights or your comfort level.
I have insanely good vision, but I can’t hear for shit. So I can descend in the dark with minimal lighting in comfort. But I might shout “What?” to your face in regular conversation.