buy the best one you can!
features like infinite wire speed & heat settings are great for when you want to dial it in to your liking (guage of metal, user experience, using weld thru primer, etc). like trevor said, availability of replacement parts is important...but also, what type of parts are in there? plastic, aluminum, pot metal??
i bought a miller 135 quite awile ago and it has been an amazing and versatile machine. quality internal parts (little to no plastic), infinate wire speed & heat settings, 110V....it's a little more expensive but i highly recommend it.
i think kms also has a try before you buy station set up.
as the great philosopher matt once said....pay once, cry once.