setup anti dos attacks

Hi, I would like to setup anti dos attacks in our Fortigate 200D. My first question is, is this advisable? I mean as far as I know (correct me if im wrong) if I set this as my primary anti ddos device and if the attacks are big, my device could end up dead. So usually the mitigation is on the ISP side or a third party mitigation. Can I set this as my secondary? Will the performance: speed be affected.

Thanks

Jeff