[Corsa C 2000-2006] Engine light and limp mode when engine hot

[Tmr2014]

Hi everyone, ok so here goes....I have been offered to buy a corsa c 1.2 16v sxi petrol on a 55 plate. However the guy who owns the car has had a problem with the engine light coming on and has spent over 600 pounds on trying to get it sorted. After visiting three separate garages to be hooked up to diagnostics he has replaced the Air flow meter, plugs, injectors, coil pack and even had a refurbished ecu fitted and reprogrammed. I changed the fuel filter for him today before I gave it a test drive hoping that it may have just been blocked up or whatever.
the car started great from cold and run smoothly with no misfire and steady rev counter. Drove for about six miles and just before I was to turn around and return home the engine shaped light came on and remained steady on before flashing about a mile later and then going back to steady. The car lost all power and was running like a bag of nails barely letting me reach 30mph much to the annoyance of the motorists behind me. I managed to 'limp' home and told him that I won't be buying the car which incidentally was up for sale for a thousand pounds. As he is a neighbour he knocked me a little while later to say that he is prepared to cut his losses and let me have it for five hundred pounds. As tempted as I am I just don't want to take on a nightmare car. I did the pedal test and came up with four codes which are : 0300, 0301, 0304 and 0340. Looking on google identifies misfire on the first three codes but the last code shows a knock sensor problem. Now would the misfire be causing the knock sensor to show up or the other way around ? Do you think I should pay the 500 for the car. It's immaculate and a shame to send it to the breakers ! Any help and suggestions you guys can give would be very much appreciated.
thanks guys

[Blackie]

Hard to say, the misfire could be something simple like plugs, or it could be the coil pack, which would be my gut instinct as the symptoms are like a faulty coilpack, the individual coils breaking down when they get to normal operating temp. See if you can knock the price down a little more and worse case scenario you make your money back breaking it

[Taurus]

[Tmr2014]

Hi guys, well thanks for all of the tips. I think I will try the egr trick first and see what happens, failing that I could invest in a new coil pack even though it's had a new one. If they don't work I think I will leave the owner keep his car ! Don't have time to break and sell as I have two jobs but thanks for all of your advice, I will post with updates.