Spongy brakes are usually a sign that the brakes need to be bled. This is an easy procedure that you can do yourself relatively cheaply or you can take it to a mechanic and it can be done for about $50. If the brakes need to be replaced you are looking at $75-$100 dollars per pair.
Disc brakes
They are disc brakes and self adjusting
Yes, the front has discs brakes on them.
Power assisted disc brakes.
If yours has the rear disc then they all have them.
Yes. I own a 2000 Chevy Blazer.
YES
You can't adjust disc brakes
My sources show there are different part numbers for spindles on the 1967-70 (drum brakes), 1971-72 (disc brakes) and 1973-87 (disc brakes).
Yes. There are manysuppliers of aftermarket bolt-on kits.
There's no guarantee that they will. Disc brakes have a higher initial cost and are prone to heating more quickly than foundation drum brakes.
NO. I had a 96 and it had drum brakes. I believe the rear disk brakes started in 1998 for the Blazers.