Design a programme for invoicing

I work for a private ambulance service and we need an updated programme in order to speed up the process of invoicing our customers - ideally a programme that can link to our calendar where we have most information relating to patients/jobs so it prevents us from having to input the same data over and over again. Not looking for anything too complicated. The programme would also need to be on all 3 of our computers in the office. Currently we are using Mircosoft Access and this is obviously an old way of doing this - we also use the Microsoft Outlook calendar for inputting our jobs. We aren't very technical here but are hoping someone can assist! Very important point is we can only deal with freelancers who can speak fluent english - Thank you!