I'll take a stab at answering:

1. Before the guy performs the R-134a conversion.
2. No, no way, uh-uh. The website you reference is for a 201 chassis car, yours is a 124. Big difference.
3. Not enough available on the web, you must have the factory shop manual.
4. Basic hand tools are sufficient for replacement, special a/c tools are necessary for charging with refrigerant.
5. I don't have those memorized, lots of good discussion on it available here using the search function.
6. It's coming off the various doors that open & close to direct the air where it needs to go. Nothing to replace really, just vacuum the remainder off while you have it apart.
7. R-12.
8. Anything that leaks. New expansion valve is usually warranted when taking apart the system.
9. Huh?
10. Beats me, I'm in Texas.

Seriously, when did you hear the hissing sound from the vents? All the time, only when driving, only when accelerating when driving? When a bit low on refrigerant these things hiss. I suspect you have a good size leak - it may or may not be in the evaporator. Best to invest some time and effort in locating the leak before diving into the dash.

- JimY