If the garage fits a part which is soon found to be faulty, do not pay for a replacement to be fitted. The garage should have fitted a good part the first time.
Tell the garage exactly what you want repaired. They should tell you roughly how much the work will cost and when your car is likely to be ready. If you tell them to check your car and 'fix everything', you might have to wait longer than expected or pay more than you expected.
Ask for a written quotation. The garage may say it can only give an estimate because, until it starts the work, it will not know what needs to be done. If this happens, make sure the garage gets your permission to continue working if it becomes clear that you are going to have to pay a lot more than was first estimated.