For 2001, what are underbody age/rust issues to be aware of?

I'm hoping an (older) Chevy mechanic has some tips on what I should keep my eye on.

example: I know carriage/cab bumper/bolt+washer is very rusty - but is there a reason to replace really?

what I already know underbody (not engine or trans, non-evap):

* break lines gas lines, rust out need replace on older trucks. (i barely got home one day when a brake line split open!)

* rocker panels & corner panel rust through due to lack of drains and bad "closed bubble that holds water" design

What I don't know is what I'm interested in. Suspension parts, underbody parts that are usually not considered regular maintenance that I should keep my eye on for rust.

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I considered downsizing this old rusting truck for a smaller older one. I find out anyone with a non-rusted older small-mid size truck NEVER SELLS IT? New are really expensive and ever more complicated to DiY. So I keep it - I guess. Though I don't need a King Cab never did.