Yeah he knows way more than I do - and I agree that all lithium is probably the best way to go if you can afford it and are starting from scratch.
In my case, similar to yours, I had a ton of lead already - replacing all of it would have cost a lot of money, and would have been wasteful as the lead was only a few years old.
> Do you think I can use MPPT controller instead of DC-to-DC charger?
Only two issues I can think of, but definitely do your own research: 1) unsure if you can set up something for the MPPT to cut off charge if your source battery drops below a certain voltage, which you can do with at least some DC-DC chargers, and 2) it is common for MPPT charge controllers to lack the capability to boost voltage, just convert it lower.
#2 can be mitigated by setting up your lithium bank to provide higher current, which has its own dangers, but has added benefits as well such as requiring smaller diameter wire.
#1 is a serious concern and can also give you safety problems outside of maybe killing your lithium - so please make sure you're being safe and doing your research. maybe ask in diysolarforum.com ? I haven't posted there myself but have gotten great info from threads there.