Any house with an original boiler from the 70s needs it changing now - whether or not the thing is currently working or not. You need to factor this into your price, perhaps allowing �2k for a heating engineer to change it.
Old boilers are not only pretty inefficient by modern standards, but the parts are going to be impossible to obtain and you can be sure it will break down in the midst of winter when you need the heat and plumbers are super busy (and so more expensive).
It's a problem just waiting to happen.
The business about the sellers being responsible for the next 12 months is pure myth - it is for you to have the boiler checked (and indeed the rest of the equipment in the house).
I wouldn't even waste the money on having the boiler checked - assume it needs changing before the winter.