Those old postal vehicles were made from the GM parts bin with a custom shell built to spec, though most of them were from the S-10 platform. They are not indestructible, the USPS started scrambling to buy COTS vehicles while the NGDV was in development.
The S-10 itself is a really good farm truck though, so I'd keep your eyes on the market for one. Or maybe the equivalent Ford one.
Old Toyotas are indestructible as well and oem parts are readily available.
We had a 1991 s10 for years. I didn't realize that's what that was based on.
I'd like to have a Hilux, because the Tacoma isn't as good but that's just not an option here.