It is impossible to answer your question with any certainty because the life of a car depends not only on how it has been maintained but also on how it has been treated by all the people who have driven it. For instance if it has been revved hard immediately after starting and done lots of short journeys where it doesn't warm up properly and spent a large proportion of its time in heavy traffic (where a large engine does not run efficiently) then the engine may be at the end of its life.
If, however, it has spent most of its time on the motorway, been treated properly and had good quality oil in it then it could be perfectly sound - an Alfa V6 is capable of 150,000 miles or more.
The fact that it has had so many parts replaced recently is a reflection of the fact that certain parts of any car have a limited life expectancy and at 100,000 many parts may be past their best and need replacing, but that does not make this a bad car. But high mileage cars are cheaper for a reason - if you don't want to have to spend money maintaining a car, buy a new one! If this car hasn't had the clutch replaced already then I expect that will need doing within the forseeable future, but this again depends on how it has done those 100,000 miles.
If you are planning on doing 15k a year I would get the car inspected - either professionally or by a knowledgable friend - to minimise the chance of nasty surprises. That also allows a more balanced, unbiased opinion than yours if you have already fallen for the car! If you are still worried about the mileage, keep looking - there are lower mileage cars around.
Whatever 155 you buy - enjoy it!