Choose best Project Management Software

I am looking for Project Management software with Gantt charting which has a particular requirement which I have had trouble locating myself. I am therefore asking for someone to research what Project Management software out there will meet my specific needs. I am not asking someone to write the software - just find what is already written and available. Software MUST have the ability to assign resources (such as staff) to a project and do 2 things: 1) Know if that staff is over-assigned in which case an alert should be shown - people can't do 2 things at once (especially if they are in different places) 2) If a person is assigned a 5 day task and on the 3rd day has a sick leave day (or has to go to a meeting, etc) then staff must be able to easily click on a calendar or similar to indicate 1 day of inactivity in the middle of the task. The result should be that the task should show in the Gantt chart as running for 2 days, then 1 day no work done on the task (preferably with info on why not - leave or meeting or other project), then 3 more days work on the task after the day off, so the 5 day task actually goes for 6 days. Software should cost less than USD500/annum for an company with 20 users (or be a reasonable 1 time price) I am not interested in a generic list of Project Management software that I need to look through myself to see if it matches my criteria. Your report must give 3 web links to show me the pricing for the system and at least screen dumps to prove that it has alerts for over-assignment and task inactivity/schedule delay handling.