Might not get you all that far. Tried the SIP with reasonably sorted wire feed and external power supply for the motor (so constant feed speed). Still wasn't all that great. Course you'll have something nicer than that (I forget what).
I suspect a add on motor and wire feed mechanism wouldn't be all that rewarding, though I have thought the same thing - they are just a bolt on unit after all.
I have'nt got a mig yet malcolm,thats why si keeps taking the pee out of me (i had to sell my last one to pay some bills about 3 years ago),im hoping to get a decent machine ,it was just a theoretical question as i know some machines dont have the strongest wire feeds and if it breaks then upgrade if possible.
But i shall moderate my questions coz they annoy Si