This sounds like a fun project ( finding a light to super wow your dad).
I am certainly not an expert in this field, but I do have 3 of the zoomies. For the most part all of these lights that use the 30 ish mm zoomable head have around the same performance.
I have one of these.
One of these
And one of the 14500 zoomies mentioned before. Maybe different brands of them but basically as far as build quality and output most are very similar.
I am inclined to agree with what djozz said about a factory zoomie probly not being too impressive to your dad (just my opinion).
Also as dthrckt mentioned the driver in these typically run about 2.1A, meaning they are under driven.
I did some testing on these, not for lumens but for lux ( throw at maximum) and across the board the average is aobut 8,000 to 14,000 give or take.
The thing I cant get past about both of the factory 18650 powered lights I got is the driver board wont stay in the same mode. Every time you turn it on it moves to the NEXT mode. It doesnt start in the same mode each time, nor will it ever memorize the last mode. What they do ( at least my 2) is if that last mode you were in was low, when you turn it on the next time you will be in STROBE :(
Not only that what they call "heat sinking" is a PCB over the top of a big empty hole.
I dont know your dad so, I'm sure you will know what is going to be best for him far better than I can, but I do have another suggestion.
How about a custom/modified light?
Those $20-30 zoomies have decent bodies and believe it or not the lens in them is competitive with some of the high end AR coated lenses of equal size.
What I personally have done with them is to start by making a heat sink for the PCB to sit on ( I have a lathe), then by de-doming the emitter this will almost double the output. Then by replacing the factory driver with a custom 7135 based driver to increase the current to around 2.8A. This puts the light in the 30,000+ lux range, with brighter light, dependable heat sinking and USEFUL modes for the light.
Again this is only a suggestion based on my experiance. If you think this is something that would tickle your dad, just give me a holler. I'd be glad to put something together for you and it would come in price wise right around your $100 budget mark.
Good lux with your search :)