Without actually seeing the boat it is hard to judge condition vs price. However, IF this boat is In as good shape as you say, well... I think you better snap her up before someone else realizes what a good price this is. $900 is not cheap", but it is the low end of average for a typical DS I in "GOOD" conditon, since they are the preferred version for Class Racing.
What condition is the woodwork in? Roadworthiness of the trailer? Condition of sails? Age of rigging? For that price I assume that the floorboards are long gone? Has the mast been modified to have a hinge just above the cuddy to make stepping/unstepping the mast easier for trailering (not overly difficult to step/unstep the mast with the original "keel-stepped" setup, but not something I would want to do every time I went sailing (if not modified yet, parts cost around $90 and installation is pretty straightforward.))
For comparison, I paid $1400 for my DS II in 1996, that included a pretty good but not perfect trailer (I replaced the axle, springs, hubs, and wheels/tires in 1998 to the tune of $400), original sails from 1979, and cosmetically, she was not bad (a few hours with some rubbing compound and a buffer that Fall made her shine!).
