Hi
if the installation still has fuses and not MCBs it is likely it does need rewiring,if the wiring is in twin and eart h PVC,I would say you can possibly get away with New Split consumer unit giving RCCD protection on socket outlets,the earthing will also require upgrading, If you just have the fuses and earthing upgraded I would say allow about �300-�350 hard to say for rewire cos you haven't said how big the flat is,
If you are going to let the Flat you should have a periodic inspection report done (it will fail without the upgrade) this report will cover the wiring for up to 5 years or untill a new tenant moves in, if you are going to upgrade ask for the periodic to be done at same time, if you have a periodic done it will cost approx �150,
hope this helps you, Ray