Well, to cut to the chase, you want static IP’s on both ends.
You can do it with dynamic on both ends, but you’re setting yourself up for a lot of headaches. You will have to notify all users at both locations to change their vpn client settings every time either ip changes.
Additionally, a dns service can work is some instances, but you don’t need a dns service that merely picks up when your dynamic ip changes, butg one that will host as static. Now your costs are rising.
In all actuality, check with your ISP, because many of them are giving away up to 5 static IP’s at no additional charge on coroproate accounts.
You would be well advised to enter into the vpn arena, especially on a site to site setup, with static ip’s on both ends. You will be facing enough obstacles already in interlacing the 2 networks without adding more to the mix.