We're planning of setting up a two separate farm (internet farm and intranet/extranet farm) for security purposes. My question is that, if we use the Edge Topology (using the ForeFront AUG) is't okey to setup now a single farm and use the AAM (Zone) to separate the internet and intranet/extranet site instead of 2 two separate farm?
You should definitely read this as described by Microsoft
Considering the security requirements, you will typically need separate farms one on the DMZ and another in the corporate network perimeter, when you are publishing content from an internal authoring farm and making it available through content deployment on the consuming DMZ farm.
Incase, your security concerns are limited only upto choice of alternate authentication mechanisms for partners/client users outside your corporate domain , you can go for a Claims or FBA based extended web application leveraging AAM. In this case a single farm behind UAG should address your requirements provided you have correctly opened the ports to your internal network for the web applications hosted in SharePoint.