I hate to say it, but I'd still question whether it's worth spending a minimum £1400 on cosmetics on a car worth £3000 in 'good condition'?
If it's only going to be kept for another year or two, I'd be tempted to leave it. After all, there's every chance that it'll be rusting again by then (not so badly perhaps, I concede) and you'll still be £1400 out of pocket.
I reckon 50% @ £1400 is about the going rate for a decent job privately anyway, considering they're not replacing any panels? Seems like a typical 'get out' from MB these days - offering to pay 50% of a full-retail (inflated) price, which means you're covering the majority of the cost anyway.
And at least by going privately you can choose who does the work for you - going by recent threads on here I wouldn't really want to pay £1400 to use their own bodyshop if I didn't have to...
Will