the age of the existing install has very little bearing on the cost of a rewire.
although 1970s the wiring could still be perfectly fine and just an earthing and bonding upgrade required. £2k is a cheap price for a rewire unless that doesn't include patching the plaster up afterwards.
don't be fooled into thinking that rewireable fuses are banned. they are not. a BS3036 fuse is still perfectly serviceable if theProspective Fault Current is low enough.
best bet is to get an electrical installation condition report ( EICR) carried out by a company who is a member of an approved body such as NICEIC, ELECSA NAPIT etc. you will get an honest independent appraisal of the condition and can then get costs to bring it to a safe standard.
as others have said, advise the buyer that the price takes the cost of a rewire into account anyway.
If you are in within an hour of E11 then PM me and I can come and have a look and give you some face to face advice