First, keep in mind that these cars for a long time were regarded
as worthless workhorses. As such the majority of them are either
one owner much loved cars, driven hard and put away wet, or retired
to a weedy field or barn for a decade or two.

There are good ones out there, depending on your definition of good
there are actually lots of good ones out there. In some climates the
insides will be shot to hell but the metal will be in good shape
(deserts) and in others they will have moss growing out of the rust
holes but the interior will be in fine shape. Unless the car has
lived in a garage or under cover it will likely have one or the other.

A fine condition 145 with M41 transmission, IPD goodies, original
paint in a good color, and so on will be worth $5000 or more, but
probably to only 2 or 3 people on the planet. The same could be said
of a perfect 71 164e or 1970 142E (or was it 71 that some few 142s
got the D-jet? (but only one or two people will dish out that
kinda money).

A more typical early 140 will likely be worth $1500 if it is in
overall sound condition, the interior is nearly perfect, everything
works, and it only needs minor tinkering. 144s are the least desireable,
then 142s, then 145s. An automatic will greatly reduce the value
though if the example is a gem it is easily enough overlooked (except
if it is a 164). For that $1500, don't expect a perfect car but
instead a car that maybe needs shocks or a new brake caliper or a
new windscreen or a bit of trim or a bumper or tires. But only one
of the above list...

Rust is another matter. It's not for nothing that people call rust
cancer. In my climate cars don't really rust but if the car you're
looking at has some visable rust you can be sure there's more
elsewhere on the car.

Often, there are 140s out there that need the whole front suspension
and tires and maybe a bit of brake work. This will be especially true
of the barn/field 140s. Don't be afraid of this stuff since these
are all easy things to fix and such, but for a whole running gear repair
you can spend $1000 or more for brakes/balljoints/bushings.

Don't be afraid of spending $2500 on one of these cars. They are
good looking, can suffer almost endless abuse and still get you around
safely and in comfort. The original bits and pieces are made out of
*very* high quality materials so a grubby interior can probably be
cleaned up to almost new as long as some nitwit hasn't cut holes in
things for a fancy new stereo. Just keep in mind that buying the car
is just the first of several payments.
chris