As a Jewish girl who was born and raised in Philadelphia, PA I feel your longing for bagels and lox. Alright, in my case, I'd prefer smoked whitefish. Having said that...
Give up all hope, all ye who enter here!
First off, you can't get a great bagel in Nashville. You can get an alright bagel, but that's about the best you can do. Bagel Face Bakery makes the best bagel by far in Nashville, and if you know bagels, you'll probably think "These are tasty! It's a pity they're so puffy...". There is a place called Noshville...the lox aren't bad, but the bagels are lousy. Bagel Face Bakery doesn't serve meals at all, and Noshville is entirely a la carte.
Brunch buffets don't abound in Nashville. Those that include Jewish specialties are simply non-existent. There is a place called Ellendale's near the airport with a Sunday brunch buffet. Their food is uneven, but some of it can be great. In my humble, your best bet would be to go to a place called Monell's. They have a location just outside downtown, and one by the airport. They do not have a buffet, but they serve their food family style. Big platters of (almost invariably delicious) food are brought out to the tables and are passed around. Everything is all you can eat.
Monell's serves Southern classics, so expect fried chicken (yes, even with breakfast), biscuts and gravy, sausage, bacon and country ham...early on Sundays they serve mostly breakfast-y foods. By early afternoon, pancakes are replaced by pot roast.
If you absolutely must have a traditional buffet, Ellendale's is probably going to be your best bet.