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.