everything jwalker says its right, not entirely sure about the gasless wire though, i seem to recall you had to use reverse polarity for some wire, meaning if the mig doesnt have that facility you cant really use that wire.
I have a sealey welder, its 160amp and OK for most stuff. The wire feed is a bit flimsy but suprisingly its lasted me 5 years of use.
Its a bugger in the wind like all gas migs and if welding in awkward places you do need to try and keep the cable straightish to avoid wire tangles.
i think the clarke proffesional range are ok as are Cebora, Migatronic are very good but probably expensive.
At the end of the day you get what you pay for. a cheap clarke wont have smoothing chokes or as big capacitors in the power supply and will give a rougher weld than a more expensive model with smooth dc output.
This clarke is quite a nice one to use(friend has one)
And i have the earlier model of this one.
Mine was £150 "round the back" as codders says