Finding an old car body is probably the least complex part of this though. There are thousands of decent bodies with junk motors in every state in the US. The barrier is batteries. Batteries are monumentally expensive, especially if you aren't buying them used from scrap vehicles.

I think it depends a lot what you're buying. If you want an 80kWh pack like a Tesla, then yeah, that's going to be $10k or whatever, but a lot of BEV conversion projects are looking more at an around-town car anyway, where a 50km range might be adequate, especially as a starting point while you get everything else working.

So yeah, if that's your target, then you can build a much smaller and cheaper pack out of prismatic cells, eg:

https://evsource.com/collections/calb-1/products/calb-100ah

So like, 20 of those is $2800 and would get you 6.4kWh (range of like 30-40km).

Renault Espace had (has?) a plastic body.