As far as the VPN config goes, yes a Pix 515E would be able to do this for you. The commands and syntax are slightly different but the principles are the same.
What i'm not sure is compatible is the PBR you are doing. On the pix you can use policy NAT which allows you to NAT to a different address depending on the destination address which would meet some of your requirements. But you are also setting a next hop ip address which i am not aware you can do on a pix.
You could just set up explicit routes for the 192.168.168.0/24 and 172.30.1.0/24 networks.
Pix firewalls also don't have loopback interfaces. Without knowing the full topology it is difficult to say whether a Pix will meet your full requirements.
As long as you purchase the right IOS with it, then yes you should be able to do replace your 1700 with an 1841.
Bear in mind that there may be workarounds with the Pix to accomodate your needs but as i say it is difficult to say for sure without a requirements list.
Generally speaking routers give more flexibility than pix firewalls especially in terms of routing. I would use a pix if i needed a dedicated firewall device. With VPN's a router can actually give more options.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...