Short: We have a church of about the same size as you, as well as me personally being a CMNA-certified Meraki partner. Personally, because I get incredible deals on Meraki equipment and licensing, I got the MX84, but depending on your guest WiFi implementation, I don’t see any issues going with the MX64.
Longer: If you’re also getting Meraki MRs (we have four MR33s and four MR42s), the guest access stuff is largely handled at the AP. We have two “guest” networks, one called “Guest” and the other called “Teachers” (I didn’t chose them). Because we can set up DHCP, rate limiting, firewall rules, and traffic shaping at the AP based on SSID for clients on these networks, these sequestered networks don’t demand a lot of the security appliance. Clients on our third network, “Office,” which ties them in to the local network, have the rules applied at the network level by the MX.
If, however, you had some other solution, say VLANs with third party APs that don’t do anything but let people on a network that then has to be managed by the firewall, your more likely to see a performance hit on Sunday mornings. For what it’s worth, we see about 1/4 of our congregation use the wireless on Sundays (been slowly creeping up, and I’m hoping for more).
That’s all to say, depending on your usual attendee subscribers, and how much of a burdon your guests are to your MX on Sundays, it’s very likely that an MX64 is going to be just fine for you. Our Sunday usage isn’t much higher than any other day of the week.
EDIT: I forgot to mention, we (day job, as a Meraki partner) frequently do 30 day trials of hardware. If you like it, you buy it, if not, just send it back. See if you can get an MX64 and however many APs you need, put them in, and see what it do!