You can definitely use an ASA HA pair to do what you are asking. Whether or not the 5545X model is sufficient depends largely on the amount of traffic being inspected and the policies your create. You should work with your partner or Cisco SE to develope a detailed design.
In general, you should think about exempting some traffic from inspection. For example, branch to Intranet traffic should not generally need URL filtering. In fact, I generally recommend customers consider Cisco Umbrella (former OpenDNS product) for content filtering vs. the FirePOWER URL filtering as OpenDNS is more flexible and has better coverage.
As far as traffic steering through the ASAs, a lot depeneds on the details of how your WAN setup works. For instance, are your private WAN routers PE routers performing MPLS tagging? If your IGP is EIGRP, are you depending on any features that are not supported on the ASA? are their other features (Netflow, PBR, AVC, QoS, WaaS etc.) on your WAN routers that might be better left on the router platform. etc. etc.
I usually recommend that we let routers handle routing and only route on ASAs to the least extent necessary to accomplish the required functionality.
