BrillAudiTDI

Hi everyone, went and looked at a car i'm interested in swapping for my mercedes its a 5 Door audi A3 2005 2.0 Tdi SE diesel, with flappy paddle, quite nice exterior wise. However, the seller has said it has a Engine mount problem which costs £70 to replace as he was supposedly quoted by his local mechanic however, he doesnt have insurance or time to go and do it. When i went to view it i noticed he was only driving quickly where he was trying to show me that "it pulls like a train" and its powerful, which it is as its a 170 bhp car, regardless of this there was a knocking noise near the engine im finding it hard to realise whether, he is trying to quickly get rid of his car and swap it as it could be a gearbox fault which would cost me of course way too much, or is he being honest which of course no will know unless i buy but is it possible i could know if its just the engine mount or could it also be the gearbox at fault or something bigger like the engine. It looked like the car had stayed quite a lot as it was under a berry tree. it has 113k supposedly even though he has no service history,when the seller had his test drive it seemed as if he was avoiding driving slow, if this is useful. Another thing, the car didn't particularly knock when changed gears using flappy paddle as its a automatic with the paddle shifts for sport mode, and neither did it make any noises when it accelerate it was mainly when the car was a bit slow, or when he started the car and began to drive it made the knock noise which was pointed out by the seller however, i must say when changing gears on the paddles it was extremely smooth, I'm just worried if it could be the gearbox? Really want to know if it could be linked to any other problem or not, any questions feel more then welcome to ask.