So, ask your vendor/open source community/consultant/tech support how data can be migrated from one site to another. If your questions cannot be answered in language you can understand/in terms that you can implement, then the solution probably doesn't offer much in the way of data portability.
Also, wrt building a complex site using Drupal, as compared to using an external company, you should look at TCO and time required over multiple years -- that allows you to compare apples to apples, as the expense of proprietary systems can often be equal to one or more FTEs. Really, it gets down to how you want to spend the money.
Cheers,
Bill
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