That bike is not a SP it is a Strada with a 888cc engine.
888cc engines only appeared on 90 SP2 models with USD forks as part of the 1990 WSK homologated rules. I believe Standard 851 Strada models didn't get the 888 engines until 91/92
On the Racing side:-
http://www.ducati.com/heritage/anni9...k/851sbk.jhtml http://www.ducati.com/heritage/anni8...k/851sbk.jhtml It does look a nice bike but personally I wouldn't pay more than £3,500 for the bike.