NAT based on port

You should be able to accomplish this simply with 2 virtuals and a snat. No iRules necessary. Something like this:

virtual ssh_snat {
   destination *:22
   snat 205.206.123.140
   forward
}
virtual http_snat {
   destination *:80
   snat 205.206.123.145
   forward
}

You would want to make sure your client's routes point at the bigip, so the traffic gets sent to the bigip and through those wildcard virtual servers.