1995 Ford Mondeo V6. BIG PROBLEM HELP!

Hi, I have recently just purchased a 2.5lt V6 mondeo with high mileage and when driving it home I noiced that it was a bit sluggish for a v6 compared to my old one and the previous owner told me that it needed a good service, I also thought the accelerater peddle was quite stiff for cable, so I looked under the bonnet and fully adjusted the cable and still with the peddle to the floor it did not fully rev so I ordered a new accelerator cable thinking this would sort out the sluggishness, I also did a full complete service on it putting in platinum plugs and the reccomended 5W 30 semi sinthetic oil. Everything was going ok on the test drive afterwards until on our way home I lost power and the engine stalled, but after restarting it I wound down the windows and heard a nasty knocking sound coming from the engine somewhere, but the engine still sounded ok if u see what I mean, the oil light inside the car flashed too but I checked the oil and it was still full, so I limped the car home it was jumpy and had no power but it still sounded ok with no backfire or spluttering of any kind, but there is obviously something broke in the engine and now with hardily any power at all, I spoke to a friend and they seem to thing that due to the high mileage I should have put some thicker oil in like 10w 40 semi sinthetic but that is not the reccomended oil, im totaly lost as to what I should do, and as for the accelerater cable its the wrong one so its not actually been full throttle whilst driven but I still think it was weak to start off with, have u got any ideas as to what has broke in the engine, why there was no power before or after and what I should do?

Many Thanks
Ryan