I am no expert at fault finding, but I certainly would take on board what the seller said had happened to the car and then store that info - as he is trying to sell the car.
1st I would check are all the engine drive belts.
2nd thing I would check is that the engine rotates and is not seized.
3rd = is Fuel pressent.?
The list is endless - make sure the battery is charged, does it turn over?
Then onto the electrics, most things are covered by sensors, so if you read up on paper clip and pedel test to find any code issues it may well give you the info straight away to find out what the problem is.
having just re-read your post and all the electrics have died, it could be a main fuse - which if it is something big it could be exspensive. - But, as daft as it is could be just a dead alternaor that hasnt been charging on it's last run and just died through lack of power - that said it could be a lose earth lead.
Thing is if you find the fault before you buy he may not sell you the car, the other thing is will he let you check the car before you buy?
please keep us informed.