Is Quake II free now

You can play Quake 1 using the DOSBox emulator (it's compatible). That's probably the closest you will come to the original experience, compared to the source ports.

DOSBox emulation usually works really well, particularly for the more popular titles, which get a lot of testing. And any even remotely modern PC should have the performance to emulate it easily. You'll probably even be able to do multiplayer (DOSBox has IPX/SPX emulation).

Quake3World has an extensive guide to how armour and health work:

When you are shot, Quake3 applies 2/3 of total damage to armor ..., then the rest is taken off health. If your armor is insufficient to cover the 'debt' the leftover damage comes from your health

So when you take damage it takes 2/3 of the total and applies it to your armour. It then takes the remaining 1/3 plus any left over if your armour was totally destroyed and applies that to your health.

If that takes your health to below 0, you die.

For example, if you have 100 health and 20 armour you can take 120 damage before you die; if you have 100 armour and 20 health, however, 60 damage is enough to frag you - 2/3 of 60 is 40 which comes off your 100 armour but that still leaves 20 damage to come off your health. As that's only at 20 you die.

The function of armour is to reduce the amount of health you lose when you get shot etc. In terms of benefit any armour is better than none, and the stronger your armour the better as it means that none of the 2/3 "armour" damage will get applied to your health.