HI Adster,
if I understand you correctly your plan is to buy 2 x MX 65 and place one of them into the Main office and one in the new branch. If yes than it will be really easy.
Once you connect them to the internet and created your organization in the dashboard and add both Mx-65 to your inventory and of course created the 2 sites after it's up to the races. Just don't forget to point them to each other so they will pull up the Ipsec tunnel between themselves.
You don't need to connect Meraki MS series, you can use the legacy switches. I have a couple of branch offices that are using their old cisco switches and it is fully compatible. The only thing that you will not see them in your dashboard.
One thing what you also can conciser with your client is maybe a dual internet connection. So you can minimize any kind of downtime in case of an issue with one of the ISP. MX65 has 2 WAN connection.
I hope this information was sufficient.
Cheers,
Greg