I bought a 10:1 helical gearbox, brand new, for under $100.00. You just need to keep checking there.
I built a 4KW alternator wind turbine, back in '77. I had an engineer do some complimentary figuring, and, he came up with a 16' dia wind turbine blade set up putting out 6HP in a 25 MPH wind. I bought a Browning helical gear gearbox and mounted it backward on the blade hub. Used a governing blade system and flew that machine for several years, until a tornado went by close enough to destroy the governing joints and throw the blades out into the brush.
You won't lose enough power to notice using helical gears. I used a GE 1800 RPM alternator, self exciting. The prop would start to turn at 6 MPH and cut in at 8 MPH. I used a relay set to close and open where I needed, and an oversized diode and aluminum plate heat sink, all on one leg of the alternator. On the other leg, I used an old variac and a 36V Lester battery charger. It would trickle charge at 9 MPH and run full charge 25A at governed speed of 23 MPH and 1850 RPM.
All this was done by "feel" adjusting things up on the tower, back when my knees would handle that.
1800 RPM alternators are used on Diesel powered generation units.
We had photos but they have disappeared over the years.