I had a Quarab a couple years ago who was "over at the knees." He had no lameness issues whatsoever. He moved beautifully! In fact, I showed him quite often in Half-Arabian Huntseat and he always placed well. He was a great horse!
As for the horse you're looking at, I honestly wouldn't worry about him/her being "over at the knees." That being said, I'd still recommend having a vet check done if you're serious about buying. That goes for any horse you're interested in, though!
Take care and good luck!