Welcome on board!
Regarding the manual, depends whether you want the paper copy. I got a CD digital copy for far less money. What I can’t do without are the parts fische that Stein Dinse publish on line:
https://www.stein-dinse.biz/eliste/index.php
Parts availability for the early Monsters is still pretty good through Ducati dealers, though some of the specific early parts have been discontinued. It depends how “original” you want it, and whether the Elefants and Date Stamps on parts matter.
If you use MotoRapido in Winchester, don’t forget to ask for the 10% UKMOC club discount. They also do mail order and are a fount of knowledge. As is @Albie on here.