I really like these a lot and consider buying one about 2 times a year. A 928 is kind of a Aston Martin built by Porsche. They are an amazing touring car. They can cost thousands of dollars to maintain if they have been abused or neglected and as stated by David, many have been neglected.
If they have been well taken care of, any of the 928 cars are reliable and will last virtually forever. Electrical issues can be a nuisance but these are mostly window switches and cruise control.
Only buy a perfect example of a 928 and you will be fine. Perfect means that absolutely everything works as it should, the paint is perfect or close to it and the interior is nice as well. It should have a complete or mostly complete service history. I would buy from a PCA member who is also a 928 person.
As for early versus late, the 2 valve cars can be a deal at $7,500 or so for a very nice example. The later 4 valve cars are even better sorted and are amazingly fast. They are a true supercar. A very nice late model car will run from $9,000 to around $17,000 with concours examples a little higher.
Remember these cars were almost 100,000 when new and service costs should run around $900-$1,800 a year.
As for automatic vs. manual, I would go for the automatic. The manual gearbox was not very good in any example besides the GTS model and even those were not perfect. It is a touring car and the automatic suits it and lasts forever and has no expensive clutch replacements.
Hope that helps!