It is my understanding that the Mannheim Orchestra was one of the first large orchestras full of professionals. The Oxford History of Western Music describes this orchestra as "an army of generals", and says that around 2/3 of the musicians were considered virtuosos.
What was the average member's education?
How were they compensated?
How were they recruited?
Did the court provide them with living quarters, and either way, what were they like?
Were these musicians considered lower- middle- or upper-class?
Are there any surviving (even fragmented) letters/journals/memoirs of orchestra members?
If anyone has the Oxford Anthology of Western Music, it may answer some of these. I only have the collegiate version, which abridges all 6 into one book.