If you're not in a rush, you could try it up for £5K with a view of accepting 4?
As for the rust, don't get gunned down on price too much for that. It's really minor. Nothing more than a sand back and repaint in those areas. A lot of mobile detailers have those fine mist modelling spray guns these days and could easily sort that.
Otherwise it's a new tailgate, door skin and wings to be really thorough about it, but not even I would bother doing that pre-sale tbh, and actually, I reckon fresh panels against paint work that old and weathered would stand out more than patch ups.
Good luck. A Golf R estate isn't a bad upgrade