I wouldn't be worried about delays arriving at IAD, but rather delays in leaving LGA. That could potentially be worse. I don't think you have much to worry since you seem to have given yourself enough cushion time.

In terms of lounge options, there are two RCCs in Concourse C/D. Take your pick... I do believe that the RCC in C concourse is closer to the BD gate, but cannot remember exactly which gate number the BD flight leaves.