Am I right in thinking that this should also accept:
My parents have property in Israel
i.e. that נכס can be sufficiently abstract/general to mean 'some property' which might boil down concretely to 2/3 houses, flats etc ?
Not really. Using נכס here would mean that they have one house/flat/any other kind of property. The sentence you suggested would probably be best translated using נדל"ן, which is an uncountable noun.