I was thinking of a SS80 project a year back but nothing happened. But I think it would not be the correct car to buy for teaching driving. However its the perfect car to buy if you want to restore and keep it.
You have not shared any picture of the vehicle, but 1985 is a good year and deluxe is the model to go. I expect there would be some rust, but from your description it does not seem to be a cause for concern. Parts are still widely available but not having a valid RC is a issue.
Talk with agents and see whats the cost/fines of getting papers updated. Why did the current owner not get the RC done? May be there is some hint there. If there is a issue with registration, say due to accident or anything, then it should get only scrap value.
A badly maintained just running SS80 (with papers) should not go for more than Rs. 20-25K however since these are becoming classics now.. a well maintained one will fetch upto Rs.80K to 1 lakh.
If you really like a challenge.. offer them Rs.5K over scrap value (any scrap dealer can give you an estimate) and see what happens. Then try to get the papers sorted. If that is not possible or too costly, then depose it off as scrap (or a donor car). That way you only have Rs.5K to loose.
Also remember even if you restore it by spending money, it will still not be as good (or efficient) as modern cars of today. It will have its charm but many will think you
for spending hard earned money on a 30 year old vehicle. Majority of Indians are still earning to live.. we are still not good at being passionate on hobbies which make us dirty our hands.
Hope I have been able to help.