what do pulleys do? what are the goods and bad's? which ones should i get to suit a daily driver + performance

 

Depending on the drive of the pulley. They main reason you want to use a pulley kit is eliminate the air pump. It under drives the alternator and water pump which provides more contact surface to the water pump pulley; which would have slipped without the air pump belt. It also a dresses issues with high speed cavitation in the water pump. The issues occur with charging problems at low rpm where the alternator is moving too slowly. The weight savings is really negligible. If you still have the air pump I recommenced spending your performance budget elsewhere. Unless you just want more shiny things under your hood.

 

In principal, oversized pulley "kits" such as Greddy reduce parasitic drag on the motor. At the same time, if you track your car or spend alot of time at high rpm, they supposedly reduce cavitation of the pump as Marek mentioned. Yet another side benefit, at least on our cars, is that when you remove the AP, it provides for slightly more belt contact than would exist otherwise.
IMHO, if your car is primarily street driven the cavitation issue is negligible....same for making any extra hp available. And there's still more risk of slipage. Although they do look nice someone recently pointed out in another thread that they don't last forever. https://www.rx7club.com/3rd-generation-specific-1993-2002-16/aluminum-underdrive-pulleys-maintainence-item-781445/
And if you remove the AP, there are some who make an idler that will maintain normal belt contact with the stock pulleys. Calling Mr. Garfinkle.

 

Do you guys have any suggestions on where to get just the underdriven (larger diameter) water pump pulley? I've read that you can find them on ebay, but I'm a little skeptical about them, since they don't typically seem to have technical data (pulley diameter, material, etc.).

Or what do you think of having a local machine shop fab one up?

 

Greddy/RX7Store/Ebay alternator and water pump pulleys are all the same size and pretty much all of the same quality level.

A previous poster's claim that pulleys are primarily used to eliminate the air pump is inaccurate - the main goal is to reduce parasitic loss and you can retain or remove the air pump as you wish.

There really is no practical downside to underdrive (main and/or accessory) pulleys on an FD - the alternator, air pump, power steering and water pump will still operate satisfactorily.