brushless conversion questions

[rcsammy]

thanks, was an intersting/confusing read
so i think a 4s battery would be best? unsure of what mah i would need though
realy all i know is that i would like it to get a 1/8 buggy up to around 45mph and atleast 15 minutes runtime(is that reasonable? i have never run brushless and lipo only brushed and nimh and i guess lipo and brushless should last longer?)

the 1/10 mamba systems seem reasonably priced, will they offer a similar package with the 1/8 monster max? or could i find somthing cheaper? the Feigao motors seem very cheap

also what is a BEC?
thanks