Choices are really to either:
Use lipo with your current Brushed ESC and motor and maybe get a Novak Smart-Stop Lipo monitor cut out (PM me if you want one of these as I have one spare for sale!)
or get a new ESC (Brushless?) with a low voltage cut out (LVC) and maybe a brushless motor too. Just changed my B4RS to BL/Lipo and it's come alive!
Maybe this, not sure about them but seems a good deal:
http://www.modelsport.co.uk/?CallFun...n&ItemID=34302