Clarke 135 or 150, would be my purchase if in the same situation, ok for car bodywork and a bit of spare power for that project which will crop up after you have bought it.
I would sort of say around £80 to £150, but you could find an £80 machine in much nicer condition than one that fetches £150, it's just your luck on the day.
A scruffy but working machine may be sold for only £50 and it's appearance is immaterial if it's working ok.
The problem with ebay is the size of the market, so getting a bargain from the site becomes more difficult.
The prices are usually better if the item is collection only, the seller has removed at least 50% of potential buyers due to distance, it also means you can see it working before paying
i was once uniformend enough to part with more than 300 for a new minimig from sip, it was hatefull thing. a coupe of weeks ago i bought a singel phase 350A migatronic with remote wire feed and watercooler for £300
it all depnends on what your after if you have the room don't say no to a big heavy machene make is pritty immaterial at you budget you'll never find the bargin if your picky but if you go for te heavy machenes you won't go far wrong