It might be only 50% efficient, it might be 75% efficient, a new unit
could be 83-96% 83% for steam, 94-96% for HW or forced air. It depends
on your type of heat and what you buy, Do you have steam or HW, you did
say Boiler, so it has water in it. At that age remove it but best is now
before you buy it, There can be almost zero dust if the asbestos is kept
DO GET A HOME INSECTION!
There are likely other hidden issues a home inspector may find.
They rent perfect but its the best few hundred you will ever invest.
sellers will often deduct repair cost of troubles found by inspectors
I am NOT a inspecor sold a home a couple years ago.......
buyers best friend sellers worst enemy!