Hardware is going to cost you far more than software on this one. You need to figure out your hardware and software at the same time. Personally, I am leaning towards FME Server 2012. AVL is basically just a sensor array, and it looks like FME server handles sensor arrays very well. Process the pushes from your field devices along with any diagnostic strings and output them as GeoRSS, GeoJson, archive to a database, send email alerts, etc. And then use an open source mapping option like Leaflet or Openlayers to display your feeds.