How much use will it get, and how thick of metal do you want to weld?
Most of the entry level units are < 200 Amps and use air cooled torches. This is fine for 1/8" and thinner metals, but when running 150+ amps (3/16" or thicker material) the air cooled torch will get hot in your hands fairly quick. This may be OK if just doing small jobs, but if you plan to weld long hours at high current look at the liquid cooled torch units.
There is a big jump in price going from air cooled to water cooled torch unit.
You will want to check what torch the welder is using to make sure it is compatible with common torch cups, and other torch consumable parts.

The older non-inverter units can be found at decent prices, but are big and heavy, and use more power to operate. The newer inverter units use less power for the same output, and are smaller and lighter.
For example, a Miller syncrowave 250 weighs 378 lbs, and uses 96 Amps @ 230 volts. A Miller Dynasty 280 (Inverter machine) weighs 52 lbs, and uses 30 Amps @ 230 volts.

If you are going to do roll cages and such where a foot control would be hard to use, look at getting a remote fingertip control.

I hear there are some good entry level machines now, but I have had my Miller Dynasty 280 DX for several years now, so I haven't look to see what is on the market lately.