As silver stated, this sounds like a big red flag. Let him know your serious, but that you would like to find out what is wrong with it first. Ask if he will let you take to a mechanic for a pre-purchase inspection. If he refuses, easy call, walk away.

 

there are loads of people in line to buy a discovery that is not running properly? i highly doubt that.

 

As everyone said... codes are a good place to start. I'd be thinking MAF, TPS, or maybe even a fuel pump.

Will it rev up ok in neutral?