If the gas tank is not rusty or crushed, get it.  Also, get the fuel pump,and
filter(s).  Not sure what you mean by underside but I generally get anything
that wears out on the van, trans, half shafts, starters, alternators, FI
computers, relays, switches, light bulbs, anything that still works.  I have
a bucket of light bulbs I have pulled from cars that has been keeping me and
half the neighbors' cars out of the FLAPS for years.

Tom
Illinois