The "usual" life expectancy will vary greatly depending on the care of the car. A well cared for 104 motor with 120k should be well within its half life - but the engine is a relatively small portion of the overall maintenance picture. You are likely to be much happier with such a car if you are a DIYer. Better yet would be to be a DIYer on an E420 - which has an even better motor with fewer congenital problems and an even longer life expectancy.