I bought this Cougar last wednesday

Hi, I bought this Cougar last wednesday. It has a 2.5 V6 24 valve Duratec engine, it ran fine and no issues with gearbox or anything. On Thursday the oil light came on while pulling out onto a dual carriageway and within a few yards the engine cut out and I was not able to restart. Eventually after being pushed to a safe place the AA guy found the oil filter loose, tightened it up and topped up the oil to just above low on the stick (thats all the oil he had with him). It started ok but within a few seconds it blew the oil right out above the oil filter. So AA man diagnosed new oil filter and seal, top up oil, which I had done the next day in the garage he towed me too.
When I picked the car up (£78) it had the ignition key in it and was running, it was so quite I didnt even realise it, but when I drove it I noticed over about 2000 revs a slight knocking, with no load on the engine ie;just cruising under 2000 revs no noise, over this when accelerating, lots of clunking. If I rev the engine while its stationary and in neutral I don't hear the noise, it only seems when driving under load? Any ideas please as I would like to try and save this beast.