I'm doing the same thing right now to my Big Sur. Disc tabs on my original fork...but not on the frame. I've gone with a 160mm Avid BB7 and will upgrade the levers to Speed Dial. From the brief research I put in, it seemed that the BB7 is the best mechanical set-up. I didn't give the 185mm much thought because I'm fairly low-key on the trails. Remember to budget for the new front wheel as well (depending on what you are running right now).
"The older you do get, the more rules they're gonna try to get you to follow. You just gotta keep livin', man, L-I-V-I-N." - Wooderson
'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur