Always hard to tell with air-cooled.
So much of it depends on the previous owner(s) and work already done. It's mostly about age with these cars...odds and ends start to fail and you may never know exactly what you're getting into
but that's also what many owners love about these...the constant tinkering and wrenching.
I would guess $3-4k a year on average, with the occasional surprise visit that may cost 2x that every few years.