If it's only bubbled under the paintwork (doesn't look that bad) is there anything I can do to repair it rather than replacing panels?
No, not really. Ultimately, if rust is there, until it's cut out and replaced with good metal, it will eventually come back. Plus, if you touch it up and cover it with paint you have no idea of what is going on under the surface. Also, once you start investing a small bubble you usually find that it's more extensive than you thought!!!!
Be very wary of cars rusting around the windscreen. This can be an indication of other rust problems elsewhere which will be very costly to have repaired/replaced.
Hmm, it's only bubbling at the moment - it doesn't look too bad to my novice eye but if rust under the windscreen is not a good sign I may have to walk away from this one!
If it's already rusting around the windscreen, this could eventually lead to lots of expensive repair work cutting out the lip where the windscreen rubber sits and replacing it. You are also looking at problems underneath which you can't see from the outside. The scuttle panel could be affected as well as the flitch panels. You are talking a good couple of thousand to have the 'complete' front end replaced and sprayed up well.
How do you tell if the rust has been touched up?
You sometimes can't. If it's been touched up well and sprayed over properly you may never know until the rust just suddenly appears. One of my previous minis had had the front seams under the headlights filled and smoothed. The spray job wasn't that old when I got him and I never realised there was a problem as he looked all clean and shiney....until one rust bubble appeared. I attempted to clean it up only to find that there was no metal left at all. It had rusted through completely from the inside of the panel.
If you are spending £6.5k that is one hell of a lot of money. I could only dream of finding that kind of money for a car of any kind.
How much do you actually know about minis? If you don't have a clue then find someone who knows what they are talking about and take them with you view any potential purchases. If you don't know already, find out where you local club is and see if there is someone there who can offer advice.
Good luck with your search and please don't jump into buying one just because it's there. £6k+ is an awfully expensive shed!!!