John wrote:
Steel - Lincoln or Miller MIG welders. Small units will struggle with
3/8" stock, but do-able with multiple passes.
Steel and Aluminum - Lincoln or Miller TIG welders.
3/8" stock - Medium-bigger units of either MIG or TIG.

You probably need to decide how important aluminum is. TIG will handle
it easily, MIG also, but more hassle with teflon liners, pull-guns,
etc.

You also need to decide your learning ability. MIG is very easy to
begin with, TIG is more difficult to learn, but nothing extreme if you
have average eye/hand coordination and are willing to practice.

You may find most of the welders at the auction are bigger industrial
units. Depending on your willingness to take a chance on
repairs-needed, you might want to pass and buy a new, small Lincoln or
Miller MIG welder. Good luck.

You might also want to consider an oil-cooled electric arc welder. These
aren't much good for car bodies or aluminium, but they're pretty near
indestructible (and even when they break, you can get them fixed), they
give 100% duty cycle, and regular rutile welding rods are cheap. Good
for fabricating stuff from heavier plate and sections, farm repairs etc...

Chris