question

I think the 18A model would be the best for you.
As you said, the maximum charge current is more suited to a 100Ah lead acid battery.
There is a pre-defined profiles for AGM batteries which is a good starting point. If your battery manufacturer specifies different settings, then a user defined charge profile can be set up.

18A would still be a good charge current for a Lithium battery. If you are thinking about using a 100Ah lead acid battery, then a 60Ah lithium battery would provide the same sort of usable storage capacity for your needs and so 18A would be plenty for a lithium battery of that size.

Thank You for your answer. My optimal battery would be to use a 100Ah Lithium Battery, would this still charge fine on the 18A model?

In terms of getting the battery manufacturers data. If it says the following:
Charge Voltage 14.6 +/-0.2V
Charge Mode 0.2C to 14.6V then 14.6V, charge current to 0.02C
Charge Current 20Amps

Would this then be the following in the app:

Absorption Voltage: 14.6V

Float Voltage: 14.6V

Bulk Time Limit: ?

Rebulk Voltage: ?

Fixed Absorption Time: 2h

I also have some concerns of the durability of these units in a 4x4 environment, what are your thoughts?

I am NOT an expert, but of the options you have listed, I would go for the 30 amp unit.

It can be deregulated to 15 amps, and it would be a better choice for your 100 Ah Lithium battery, which probably has a recommended charge level of 50 amps. So, 15 amp charge while you are still using lead acid, then 30 amps when you switch to Lithium. (which would be my recommendation)

By the way, do not use a float voltage of 14.6, especially not with lithium - check battery specs.