One thing holding me back is concern over maintenance and repair, this given the factories poor reputation in supporting older models and the dearth of experienced Morgan dealers.
In fairness, it is really only the S1 where I have heard "that part is no longer available" with respect to AND even then, it has only been for a few parts that were unique to it. Furthermore, and as has been said on this thread, a good engineer/mechanic will find their way around even these issues.
To lump all the Aero 8 models into one group would be unrealistic. The Aero production ran for nearly 20 years (slightly longer if you include the Aero based Plus 8); so comparing parts availability of a Series 1 vs a Series 5 is always going to lean in the favour of the later Aero 8 models.
Other than having to wait for body panels - my understanding is that they are produced in blocks, as needed - I don't believe there have been parts that are unavailable for the Aero 8 S3, S4 or 5. Most of the bits that could need replacing were bought in and can be found either via specialists (such as Melvyn Rutter) or MMC themselves.
However, I am hoping to start working on a project which should help Aero 8 owners moving forward. It will take some time, and isn't top of the list in terms of priority but it is on my ToDo list!