To attach the seat to the bracket, there should be a threaded insert within the seat. So no nut just like java said. The typical bolt is a 12.9 class.
As far as mounting the brackets to the floor frame, factory seat bolts are 8.8 low head socket head. 10.9 or 12.9 are a bit hard to come by in low head type. Even a metric specialty place like MMCC doesn't have the higher property classes
http://www.metricmcc.com/catalog/Ch1/1-48.pdf
But the low head bolts are only req'd if you intend to use sliders- for the sliders to pass over/clear the bolt head. So if you're fixed-mounting the seats, the full head bolts will suffice and the higher property classes are readily available.
BTW I used 8.8 class on my Cobra race seat with sliders. I'm hoping that's sufficient since the stock seats are bolted in such a way. Albeit the factory seats are mounted with 6 bolts per (2 front, 4 rear) instead of just four like i've done with my race seat.