Hi, I have owned several Wadia separates and they were all incredible. But for your question, the oldest is the 6i, then the 21 and the 830, 850 were produced during the same period. If I am not mistaken the 6i, 21 and 850 all use TEAC based transports, the 830 has a PIONEER STABLE PLATTER (reversed label). The difference between them is the resolution power 18 to 22 bits I think.
As far as buying an older model, I would recommended the 16 (not in your list) or 850/860, they were the top machines made by Wadia in the past.
You could also contact Steve Hunley at Great Northern Sound, he used to be with Wadia and now upgrades/services them. He has great knowledge of the older products and is a very nice person to deal with.
Good luck
Serge
As far as buying an older model, I would recommended the 16 (not in your list) or 850/860, they were the top machines made by Wadia in the past.
You could also contact Steve Hunley at Great Northern Sound, he used to be with Wadia and now upgrades/services them. He has great knowledge of the older products and is a very nice person to deal with.
Good luck
Serge