There is a reserve lock to prevent damage, ie, forward, brake, reverse, so a slight pause after braking before reverse will engage is normal. May need to reprogramme Failsafe though?
Re batteries, trust me on this, once you have gone Brushless (which is exactly what I did, went from brushed on NiMh to Brushless on same) you will want to go for LiPo almost immediately, the speed will become intoxicating! So just get NiMh batteries appropriate for your brushed and then go whole hog and go brushless and LiPo, pref 3S - it gets everything very hot in an otherwise standard Carnage but it will shift! See my posts with my now 4S HPI Trophy which I am looking at potentially upping to 6S and I started with a brushed Vantage and a commitment to spend NO MORE money than that......