If it's also called just the Hobart 300, the owner's manual is available as a .pdf file (2.4 MB) through the "Owner's Manuals" link at the top of this page. In the alphabetical list, find "Hobart 300."
Also, do I hook up the wirefeeder in reverse polarity for mig vs. flux core?
Will my power supply work for arc or tig?
As a general rule, use DCEP (electrode positive, reverse polarity) gor GMAW (MIG) and DCEN (straight polarity) for FCAW (self-shielded.) If there is no switch on the machine, you'll have to manually swap where you connect your leads.
If it's designed to be used with a wirefeeder, this is a Constant Voltage power source (CV,) right? To be used for Stick or TIG, it must be capable of switching over to Constant Current (CC.)
All of the above processes are types of electric "arc" welding.
The wire is real rusty so I'll go ahead and replace it. Would .035 be good for all around welding? I'll be dealing with .120 wall tubing and 3/16" plate mostly and I'll be using it as a mig and not flux.