Brushless models costs more initially but the difference in speed is amazing. Converting a brushed to brushless is a separate and additional cost .. but also usually worth it for the benefit of speed. Brushed has its place - ie: 99% of the crawler market is brushed. Plenty of other vehicles also use brushed motors. You will generally see repair costs increase with brushless - more speed, more stuff breaking.
For batteries, the 2S (7.4v) LiPo is generally safe. Special care does need to be given to using, charging and storing LiPo's - a fully charged LiPo is a fire waiting to happen. I would always choose a hard case LiPo over a soft case LiPo. The higher the 'S' (2S, 3S, etc), the higher the voltage / power, the higher the LiPo danger and generally speaking, the vehicle repair costs.
If you mean spec racing, Arrma is no good for spec racing - they're bashers, not racers. TLR is good, but anything race spec is going to be expensive - and most you have to build (which is a major plus imo). Spec racing uses soft case LiPo's.