The previous advice given by all is right on the mark in my opinion.
I used to do ornamental iron work. Do yourself a favor and buy yourself a 220v Miller. Yeah, blue is expensive...
One of the main things you need to look for in a machine is the duty cycle. The percentage listed is X% of 10 minutes. So, for example if you find a machine with a 20% duty cycle (Harbor Freight), it means that it can do 2 minutes of welding then it needs to cool down for 8 minutes. It needs this to prevent it from overheating otherwise it will die an early death. I don't know about you, but standing around waiting for a machine over and over gets really old, really fast...
The 220v machines will have better penetration on heavy stuff. Stick with the bigger brands and you will be very happy with performance. Also, most importantly, parts are readily available from your local welding shop.
I'd also recommend using shielding gas rather than flux core wire. It allows more flexibility with what the machine is able to do.
If you plan to do aluminum, then try to find a unit that includes a spool gun. You don't want to buy a spool gun later as it's expensive to get a good one. Aluminum wire doesn't push well through a long mig lead, hence why you need a spool gun. Plus, if you run a standard lead for steel and a spool gun for aluminum, then you don't have to deal with cross contamination issues.
Finally, I agree with watching Craigslist if new is out of reach. As recommended above, try to buy a package if possible.
Good luck!
Steve