Potential Jimny Owner but how much rust is too much rust?

If and it's a BIG if, that's the best one you can find then it might be worth the time and effort however if as suggested it would be done professionally don't expect it to be cheap. Personally I would keep looking, those pictures are not inspiring me with any confidence. Are you absolutely sure about wanting the soft top as I would be putting budget towards finding a newer cleaner tin top.

I would be tempted to say there is some body mount rot on this (front slam panel). It seems like it's had an unprotected life. However, the rust doesn't seem too bad (going off my 1999 Jimny that I resurrected from rust). However, I read you are perhaps not so handy - a Jimny of this caliber may fix that and get you working on the underside to tidy it up a bit. Scimike points out some very good areas to look out for! 

Whereabouts in the country (roughly) are you located?

 

As others have said, you have to look more closely to find most of the real problem areas and that's how they can sometimes get through an mot.

I think mine got through by the rust being covered in mud from off roading, then I did a body lift and found all of the above. Another one that you can check easily is the rear foot well if you peel the carpet down from the edge or the rear seat.

Cost over a grand to get all these bits done on mine, probably not worth it but it was to me