Alternator troubleshooting

[Dr Shuffles]

'95 TI, M42, manual, 208,000 miles.

Symptoms: Idles kind of rough. Lopey sound, like a misfire. An intermittent grinding noise under the hood. Battery voltage at 12.95V when idling (Bentley says it should be 13.5 to 14.5).

I'm pretty sure I need a new alternator, but I wanted to get a second opinion. Also, I see 70, 80 and 90 amp alternators for sale that all supposedly fit this car; how do I know which one I need?

[Dr Shuffles]

There's a sticker on top of the alternator, and presumably it would tell me the amperage, but it's so worn that all I can read is "Valeo - made in France". Does this mean I have the 80 amp? Because it seems like the only other option is the Bosch 70 amp.