This scares me even more. 13lbs is bad enough.
There is a reason that he is selling it. He knows something is up and isn't willing to replace the motor. It is easier to dump it on a kid that knows nothing.
x2 especially at a good price. If you see it take it for a test drive. turn radio off, listen for any noises, squeals, ticks, etc. Ask him to let you get on the throttle pretty hard. Check for overheating, any smoke coming from the exhaust, smell of burning oil, etc.
And get a list of EVERY mod done to the car. Ask about the fuel pump, what size / brand injectors, what brand map sensor he's running, what wastegate, turbo, tuning, etc etc. When was the timing belt & water pump replaced? What spark plugs are in there? Have him do a compression test - in front of you.
I would be VERY cautious on this now that I saw the video. A turbo car means it wasn't driven like a grandma, and 16lbs of boost on stock internals not being driven like a grandma most likely means this motor is close to the edge of kablammo.
Sounds like a good price now, but once the motor goes it won't be so good. I don't mean to be hard but I want to help you as much as I can.