I was a long time advocate of my VMNG300 and only removed it as I was on the SH2 trials. My SH2 is in Modem Mode and I can't notice any difference between it and the 300, only difference being 6 DS channels on the SH2.
I doubt if VM will provision an old 300 in any case, best just to get an SH2 and use it as a Modem.