I'd recommend a second opinion on the work quoted. When was your Cambelt last changed? It should be every 5 years or 120k which ever comes first.
Did the car have a diagnostic check before the oil sensor was replaced?
How long ago was the sensor changed? If it was within a month then there should be some comeback with the original garage.
What's the service history on the vehicle in general and how long have you had it?
Well maintained cars can be reliable, the mileage shouldn't worry you. For example my A6 has done 344k on the original engine and gearbox which have never been rebuilt. I'm the second owner, my dad being the first owner. It's been reliable but bills are expensive. As all parts wear out.
You have two choices really, trade it in and buy another car. Or repair it and run with it.
Don't forget the age old saying better the devil you know....