I think I found the one you are looking at. It does appear to be an XS400 Seca DOHC. If I am looking at the correct bike, I am betting that it is not in running condition given the price, but it does look to be complete from a distance.
As far as other information goes, there is a factory service manual on this forum that you can download. It is pinned to the top of the Garage forum. If you have other specific questions, the search function works pretty well and you can always post specific questions that owners of the DOHC bikes can respond to.
And... if you are thinking about purchasing the bike, good luck!