how to select the isp outbound interface

Sorry, my initial answer was for another questions.

Not sure if with advanced routing module (ZebOS) you can do policy-based routing, anyway that requires addon license. However, looks like you can do that easily with iRules, but I never tested the nexthop command.

These solutions together should allow you to write the irule for your requirement.

https://devcentral.f5.com/wiki/irules.ip__client_addr.ashx

https://devcentral.f5.com/wiki/iRules.nexthop.ashx