Sounds like valve guide seals (at least).
If it's misfiring at low revs then performance will be down. Why it is misfiring could be almost anything.
Try to get a drive of another one to see if the lack of go at low revs is a fault with this articular car, or just a characteristic of the model.
If you go ahead with this, adjust your offer price to allow for some engine work. Wether the engine work is major or minor can't be established without some investigation that I guess the owner wouldn't want you doing
If the price isn't right to allow you to do some fixing, walk away, there's plenty more out there.............