"how many leads were Marketing-qualified with a specific Lead Status"
To pull a straight count try a smart list with the filter that looks something like Data Value Changed: Lead Status, New Value is "qualified value", Old Value is not "qualified value". You can add a date range to this filter as well. In a separate smart list this would return all the leads that fit this criteria all time or within the date range but will not show trends.
For trends you will have to use the same smart list filters in a report smart list and then group by the date such as SFDC Created Date. That date will not indicate though when the leads actually became Marketing-qualified. For that you will need to summarize on a custom date field that has a timestamp of when a lead became Marketing-qualified. Also If you have built this date a simpler smart list would be just Marketing-qualified Date is not empty.