1)a buffer amp has no voltage gain, but has current gain
2)what you set as the output impedance with a series resistor depends on what you intend doing with it. a 600 ohm output impedance was the standard for audio many years ago because a lot of audio test equipment was designed for telephone systems. 50 ohm is pretty much standard for most test equipment these days. in the test equipment industry, the same 50 ohm attenuators, splitters, etc... are used for RF and audio equipment, so it makes sense to have test equipment all meet the same standard. 75 ohm outputs are standard for broadcast and CATV equipment. some computer monitors used to have "50 ohm" bnc video connectors but the input impedance was 75 ohms.
for audio it really doesn't make much difference, until you start driving low impedances with the signal generator. most inputs on audio equipment are terminated with 10k, and line level outputs are 1k impedance, with the concept here being "voltage matching" where 1.5Vrms is the "standard" line level.
3) a diy buffer stage isn't difficult to make, or expensive.
4)most solid state amps (i.e. those that drive speakers) have output impedances measured in milliohms. most of the low impedance is due to negative feedback and the open loop gain of the amplifier. if you missed the class, i wrote a paper with experiments here: **broken link removed**
5)it's actually lower than that because of feedback, but because of the 22 ohm resistors in the output (paralleled they would be 11 ohms), the amplifier begins to run out of steam if the load impedance goes below 10 ohms.
6)use either, but thermally couple either the diodes or the Vbe multiplier transistor to the output transistors' heat sink