If the "clatter" comes out during start up, that's the possibly the timing chain and tensioners. Normally this isn't changed on the Sentras engine and if it makes noise then better to have another mechanic take a look for a second opinion then replace the tensioners and timing chain if need be. This is due to lack of proper maintenance by the previous owner such as not changing the oil regularly.

What's the mileage of your car? Are the service records intact? Have you done an oil change already on your "new" ride? If not, pa-change oil, change air filter, transmission fluid, coolant, fuel filter and clean MAF (Mass Air Flow) Sensor w/ electric contact cleaner muna. General service. Baka matanggal pa yung start up clatter with the change oil.

Go to another mechanic/shop also for a second opinion. I think he's screwing you for your 15K. First thing, the Nissan engines have hydraulic valve tappets which are self adjusting and its very quite unlikely it will need replacement of the "shims". My 1993 Sentra 1.4L is still running on its original timing chain, tensioners and valve clearances are still okay (regular change oil lang talaga katapat).