Does your customer expect to have more than the dozen or so iOS devices, or is that it? Because if it's only ever going to be a dozen phones, truthfully Jamf Pro is probably overkill. I would never normally steer anyone away from the product, but it's a robust tool with lots of features that you just may not need, but you will end up paying a fair amount for those unused features.

Jamf also makes Jamf Now, which is much more affordable and simpler. It's geared toward SOHO environments that have a modest number of mobile devices (or Macs) that they want to manage with settings and push some apps from the App Store, etc. I would at least take a look at that. You can set up a free Jamf Now account and they give you a couple of seats for free, so you can test it out at no cost.

Regarding Apple Business Manager, that is never a requirement for any MDM product that I know of. But it's a good idea to have it if possible as it really helps automate the enrollment and setup of those devices when they are new or get wiped and re-setup. Even if your vendor doesn't participate in ABM, you can look at getting a Mac and the Apple Configurator 2 utility to manually enroll them into the program. https://support.apple.com/guide/apple-business-manager/add-devices-from-apple-configurator-axm200a54d59/web 

I agree. Jamf Now (or perhaps Apple Business Essentials) is all you need. If I’m not mistaken, Jamf Now has the ability to upgrade to Jamf Pro later if necessary. 

Provisionally enrolling devices is relatively straightforward, and as long as users don’t unenroll them during the provisional period, they will behave as if they were purchased from Apple or an authorized reseller after 30 days.