The problem with salvaged lead acid is that lead acid is not very good even when brand new, and a salvaged battery is far from that. They may be good for initial testing on a flat surface if you can get enough of them, but don't pay anything for them, and don't waste lot of time installing them if you cannot make the mounts fully compatible with the future lithium installation.
If you do want to try them, get as many as possible to make a large pack. Both Peukert factor and Internal Resistance do horrors to very small lead acid packs, and old/poor batteries take most hit from them. (Qiuick explanation of the terms: Internal Resistance causes voltage sag under load (poor performance). Peukert factor causes the battery to run empty of Ah's much before the nameplate value, when under high load (poor range).)
We currently have 9 pcs of 12V 20Ah SLA taken off from a standby voltage after a few years, so 2 kWh "nameplate", a very small pack. The had proper open-circuit voltage and didn't need desulphating. Still, they have pretty hard time moving a 800 kg car, but for a free installation, maybe worth it. I can imagine that a 10 kWh (nameplate) pack of such free batteries would be quite usable -- you might get 2-3 kWh out of it.
If you are going to do the desulphation to supposedly dead batteries, try to do it in the place where the batteries are, because I'm pretty sure only a small part of them will survive.
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