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.