Welcome to Skyline ownership. I assume you are from USA.
Please browse around these forums, there are decades worth of information here. Everything you asked has been answered.
You can start in the RB26 dyno thread and see what power people made with what mods.
I remember there was some chatter about knock-offs ending up in OE parts bins, mainly in Middle Eastern area (so maybe partsouq got affected?) Hopefully that was only a rumour...
Back in the day you could get taxi style full size and slimline ones. These days, no chance. You'll have to hit up wreckers to maybe get lucky once a year and find a car with them attached.
The projectors in them will be shit. Have you checked aliexpress for similar housings? They might be cheaper.
If I were to get those, I'd split the housing and replace the projectors with a good HID one.
Make your own.
Need to form/make housing for two seal beam units (low beam and high beam).
Or buy cheapo projector units off aliexpress, split apart, replace projectors with proper HID ones, or Koito LED projectors.
Yeah should be. I'm pretty sure the $20 Kenwood/Pioneer one sold in shops are generics, because I bought two for separate installs (one Kenwood and one Pioneer apparently, about a year apart). Both were identical with the same wiring. Worked fine with Sony head units lol.