Peugeot 307 engine cuts out when car slows down!

Hi,hope someone can help. I bought my first Peugeot 7 days ago from a trader and all was fine until three days when the left indicator stopped working which thankfully rectified itself the next day. Unfortunately for me something much worse was to follow, on day 6 I filled up with petrol at my local supermarket then whilst driving the battery light and stop light came on and the engine cut out completely! Bizarely the car started up again straight away when I restarted it so I thought maybe I hadnt put much power on the accelerator and stalled it but unfortunately it continued to cut out everytime I slowed down at a corner or roundabout. With the car to unsafe to drive I got a breakdown recovery to check it out,and they found that when the car is running the engine vibrates dramtically which they said isnt right and mentioned something about it might be the coil, but couldnt diagnose the exact faults as they didnt have the electronic equipment. They advised me to return the car from where I bought it to get my money back,as they seemed to think repairs would be costly and the faults were likely to have been present when sold to me. But since I have tried several times to contact the seller, it has become obvious he doesnt want to know, and I'm scared I will be stuck with this car, I carnt drive. My question is, has anyone experienced these problems and how much is it likley to cost me in repair bills? I dont have much money left after buying the car for £2500 a week ago, so i'd rather not pay for diagnostics etc at Peugeot if the repair bill is likly to be £300+.
Any advice on this will be much appriciated!