Just picked up a 2000 Ford F-150 xlt extended cab 4x4 5.4l triton gasoline v8 with 148k miles for cheap, but the 24 gallon gas tank leaks super bad. I am already getting a brand new replacement for $135 with new straps ($1400 from ford ), along with a new sending unit and pump for another roughly $200. It appears to be just a bare silverish metal tank, maybe it has a clear coating, I couldn't say.

So, would you just slap it on and call it a day, and be good for 10 more years? Or would you spray it with a rustoleum type paint, or maybe a thick durable brush-on or spray-on underbody coating? Would said paints or coatings actually just eventually crack and hold moisture resulting in faster rust development? Any thoughts?

I would give it a couple coats then in a few years do a couple more coats especially if it's in the salt/snow every winter

It is likely a galvanized finish. Install all the pump/sender & coat it with undercoating spray. Install tank & spray the straps in vehicle installed. Done deal.
Just don't get fuel on the finish, it'll make a mess. But the undercoat will stick the best to the spectra replacement tanks.

Appreciate the suggestions guys, and yes it will see a lot of snow and salt here in Ohio. I'll try out the undercoat spray then. Fairly cheap insurance/longevity I would think also. Also avoid getting gas on the coating during install especially while still curing. Thanks for the advise.