Hi,
NAT mainly changes source/destination IPs and mostly we use while accessing Internet or for hosting some server on the Internet. If there is no such requirement then I doubt that you will need any NAT for the VPN traffic.
YOu mentioned "Since my existing VPN are not using NAT" . So if you are not using any NAT in the existing setup then it should not be needed in the new setup as well, except if you use any overlapping subnets .
Please check document, if you already have VPNs up and working between HA to other branches then you will need some more changes to get branch to branch communication working. Consider HA as HUB here.
Thanks,
Vikas