1) How "live" is it? in other words i live close (within 40 miles) of Detroit Metro airport and if i am tracking planes within that range are the updates like seconds apart or like 1 min.?
Flights picked up by your local box are truly live. Update rate can be set within the software so users of slower machines can reduce processor load but most people run at 1 sec or 2 sec update times.
2) If i buy the radar box do i still need to pay monthly for software of some sort like airnav live flight tracking?
RadarBox has a built in network feature which allows you to view the data that other RadarBox users are receiving if they have selected to share their data (most do). This data is viewed with a 5 minute delay for security reasons. The first 12 months of network access are free after which it costs €5 / month.
For 'local' data everything is processed by the box / software so you don't even need to be connected to the internet and don't need any other software.
Hope this answers your questions.