If you buy an adventure, you will have everything you need to run that adventure. Any monsters that come from the TOB should be in the module. No extra purchase should be necessary. There are some weirdness caused by IP laws. A KP adventure is likely to use the SRD version of a common DnD monster, and have KP-specific art, unless they have worked out a licensing deal, but you should have a workable statblock for any monster in an adventure.