a- No personal experience with this machine but a look at millers site indicates it is a DC constant current machine. At least stick weld steel (and other materials with appropriate rods) probably more.

c- Ask the seller to demonstrate the machine, you should see it weld at a low setting and at a high setting.

b- No big leaks, no big smoke, be a little flexible, it's only $200. It should start without too much trouble (needs starting fluid is a bad sign). No mechanical rap when running. The demonstration welds should look nice, but that may just indicate how bad or good a welder the seller is. At the low setting the arc should start easily. At the high setting the engine should be able to carry the load without losing a lot of rpm's.

Lots of different people come here, newbe, artist, hobbyist, and pros too. You will find that you are welcome here.
Bill

Like Bill stated, "Welcome", if you find you need to ask more questions after you've had a chance to ckeck out that welder, come on back.
Many, many different levels of expertice and many different backgrounds in the welding field here, most of us fit in somewhere. :)
John Wright