How specific should a Provisional Patent Application be?

The question that you are effectively presenting is whether or not you should try to patent the combination or the sub-combination - that is, the overall system, machine, whatever, or an important part of it. That would definitely depend upon what your invention is and how unique, for example, any one component is. If you believe that one or more of the components is unique, novel, and unobvious, then you can patent the component. On the other hand, or additionally, you can effectively incorporate the sub-combination into the combination and get a patent for the overall system or machine because the sub-combination would effectively be good enough, that is, patentable, so as to effectively carry the overall invention so as to warrant a patent being issued.

As for the content of your provisional application, your provisional patent application (PPA) should ideally be as good as your non-provisional application (NPA) because you are going to need your PPA to serve as priority for your later-filed NPA or PCT application - if your disclosure is not really that good, if it is not really enabling, your priority will likely be denied and your effective filing date/priority date will only be that of your NPA or PCT application. So your PPA should effectively be as complete, as accurate, and thorough as your NPA. Think of it this way - say, for example, you simply disclose the component parts of a system, but you really don't explain how the component parts truly cooperate with each other, or why your system is a significant improvement over the known prior art, when prosecution of your later filed NPA or PCT occurs, and an examiner rejects your claims, and then you want to argue why your invention is significantly different/better than the prior art, you will not be able to refer back to your PPA for such support, so if another application has been filed between the time you filed your PPA and your NPA or PCT application, that applicant may get a patent and will have priority over your application because your priority date from your PPA will not be accepted and you will only get your priority/filing date of your NPA or PCT application which will be later/after the other party filed their application for basically the same invention. And while claims are not necessarily required in a US PPA, they are highly preferred and effectively required, for example, in Europe, so it is a good idea to in fact insert claims.