balancer is good to have. but it really depends on the quality of the pack. For the first 3 years of me using lipos, I didn't have a balancer. when I finally did get one I found out that my Orion Platinums were only off by .03V. Which is amazing for 3 years and over 150 cycles. My cheaper Blue lipos needs to be balanced every 5-8 cycles and my turnigys can usually last 15-20 cycles before they are off more than .02V..
as for your question for how many cells... it all depends on how fast u wanna go. for sanctioned racing then its only 2S. if you want to go ballistic, then 3S+.