how much would you want for the remaining drivers?

I would do $5 for each plus $5 for shipping assuming you are in the US. I have a paypal address if you want to send the money there if that price works for you. So $20 total for three and shipping.

anyone know what drivers will work with the first generation board? i burnt mine out doing something stupid.. i was fault finding and and i messed up the orientation of the drivers, they're very hot and don't respond to any of repetier commands

They are not standard drivers. They are custom made with the current control hard wired and they are quite a bit larger than standard drivers. In short unless you buy them from XYZ there are no subs. Here is a pic showing the difference. I have swapped my main board out for a RAMPS so I am not using my main board nor the drivers. I can only be sure three of them work. The extruder one might be bad as the reason I replaced the board was that my extruder was getting too much current and over heating to the point of failure.

The one on the left is a Da Vinci Driver.

How do you know your Z-Motor is failing?
NEMA 17 motors pretty robust.
Mechanical linkage tight?
Driver?   Driver current/voltage setting ?
Have you measured it's travel va command
with a dial gauge.

??? My car is going slow --- replace the engine???

Not really the same, an engine is a couple of grand. This is just ten bucks.

A more accurate comparison to your analogy would be changing the main board for a failing motor.

I would suggest that the OP does check the screws in the coupler that connects the motor to the screw. Mine came loose (mental note to get blue loctite) and it made it look like the motor was skipping and stalling. Once tightened again everything is fine.

??? My car is going slow --- replace the engine???

It is just a common off the shelve 1.8 degree nema 17 stepper. Check out ebay they are about 10 bucks a piece.

Thankyou