everyone has an opinion.. I had an sip topmig 150, wire feed was not good but put a steel liner in and modified wire feed tensioner slightly and it was a good reliable machine.
Personally I wouldn't. I paid £50 for my SIP 130, bought as an experiment to see what all the fuss was about. When my proper welder was out of action for a week it was called into use, but it wasn't really usable for thin metal so it went out of use again fairly quickly.
Clarkes work well for those on a budget. I'd not mess around on the dark side for the sake of saving a hundred quid or two (and this is coming from the person least likely to spend a couple of hundred quid).
If I didn't already have one, and knowing what I know now, I'd take a punt on it for £40, knowing I'd have to spend money on a few mods to make it work OK. I would never, ever buy a brand new SIP MIG again, though
So if you're comfortable with doing the mods and replacing any worn-out bits (liner, etc) then it's probably worth £40, knowing it could easily end up totalling £100 by the time it's running properly.
dont do it, try and fine a secondhand or new Clarke 130 or 150 amp welder instead. They are cheap and do a great job for the money. Buying something and then spending time and money modifying sounds like bad idea to me.