Regarding using LiFep04 batteries for engine starting. I have sucessfully been using a Shorai 36ah equivalent battery as an automotive starting battery for over 1 year with zero problems. The battery doesn't have a BMS and charges with no modifications to the charging system. Lithium starting batteries we're primarily introduced for motorcycles and race cars but as others have mentiond there are many advantages over lead acid and the costs have come down quite a bit.

EarthX motorsports makes several LiFep04 batteries that have a BMS and cell balancer. These batteries are designed for high amp current draw (unlike the K2) and are designed for engine starting.