Chuck,

This answer largely comes down to how much do you want to spend to avoid some inconvenience.
The most bang for the buck is a toss-up between flooded 6V Trojans and the FeLiPo4.
The cost interval between the versions of Lead/Acid makes them all about equal.
The only person I know of with Lithium only has about 5 seasons on that bank and it is still doing OK.

If you elect to go with Lithium, there are charge controllers that are very available to make them compatible with the existing systems.
You still might choose to replace the PD with one specifically calibrated for that service.

I have not done any research into the Trojan Lithium, but I am going to start. I like Trojans and have had good installations with them.
I would avoid going to a pair of 12V jars in parallel. This is a type of installation that has been the cause of most of the battery explosions that I have seen. (I like them actually because the owners insurance pays me a lot to do the clean-up and repairs.)

The I-E profile is the dis/charge curve of the bank. LiFePo4s are very different and given their choice, they could try to grab enough juice to take your alternator at least off-line if not out all together. This device will also prevent them from smoking the whole harness if there is a unprotected failure.

Hope this helps.

Matt