I have driven VW diesels since 1976. I currently own four of them. One has 330kmiles, and two are about 276kmiles and one is about 150kmiles. Normally maintenance is less than $0.10/mile (US$). I do essentially all my maintenance, and use premium oil, filters, parts. Rear brakes are the most often needed item. The alternator gets replace about every 2 years (we run the AC fan on high in the summer, and seats and windows in the winter). Generally, I do a timing belt every 100,000 ($500 parts plus labor, yes I know it can be done for less, but I service other things at this repair). On some cars the turbo can last for 330+ thousand miles, but I have one car which is on it's third turbo. One on it's second injector pump. These are bigger ticket repairs, and cause some people to dump cars.
The older Mk4 (such as the ALH) are very economical, and VW diesel friends try to pick up these because of that. I know of several ALH engines with more than 600K miles. One is owned by a woman who does no car work, and takes it to a local garage for all the work.
In general, when picking up a new to me vehicle, it takes more maintenance the first year or so. Even new cars. Then there is a steady state. Eventually things wear down, and as an example, I am replacing much of the front end on two of my cars this summer.
These individual variabilities make it hard to give you a reliable cost per mile, even with a sample size of four.
A front end I find unacceptable, may be just fine for someone else. If someone is hard of a clutch, that can add considerable expense.
Good luck as you try to analyze what to get.