Regarding you specific question, Veggie, as long as the alternator is set to charge at a normal wet lead battery voltage, you should be able to use LFP batteries without any change. The BMS will disconnect charging once the cells are full, and will disconnect the load when the cells get too low. A BMS failure has resulted in battery destruction.
DIYsolarforum is a good place to watch for the best deals/reviews on lithium batteries. I think the waiting game is a winner. There have been plenty of issues with premature failures of lithium cells, and both price and performance keep improving each year. About $500 for a 12V. 100AH battery, twice that for 24V, 100AH.