Purchasing a used car what to look for

From the sound I would say you will need a new reservoir and new power steering pump. It sounds like it has been run low/dry a few times. I would also remove the serpentine belt when the engine is cold and start it to be sure there are no other noises, in the video it sounded like it could have a loose bearing in the bottom end but that could be an accessory rattling as well. It also looked like it was misfiring or running rough a few times.
Has the timing belt and maintenance been kept up? If you are really unsure see if the owner will let you take it to a shop to be looked over by someone who is not involved with the sale.

After that the decision is yours.

The best approach is to ask to have the car for a few hours, then take it to a shop of your choice for a thorough safety inspection. This takes about an hour, plus extra time to diagnose the cause of the leak.

Most commonly, the owner is selling the car because they know what the cause of the problem is, and it will cost a lot to repair, or they tried to diagnose it already, and gave up. If the repair was easy or inexpensive, they would have done it to make the car worth more, or they would not be selling it.

Do not let the seller tell you where to take the car, and do not take their word that an inspection was done already. The shop they insist you use could have people who are friends with the seller, and they will intentionally overlook some issues.

Expect the car to be due for regular maintenance services like an oil change and transmission fluid and filter change. The seller is not likely to keep up with that if they plan on selling the car.