Hi Josh,
You're probably looking for more peer responses to this but I thought I'd through in my 2c.
If you choose to switch to Milestone, you can keep using HikVision or switch to another option if needed. If you want to verify supported models or browse other supported cameras/encoders, you should check our supported hardware list.
The best software version to start with for 48 cameras or less is XProtect Express+ as it is the entry level of our paid product versions, supports add-ons like LPR, access control, and other 3rd party integrations. It is also able to be upgraded in-place to XProtect Professional+, Expert or Corporate with nothing more than a license change. For very small installations you can even start with XProtect Essential+ which is free for up to 8 cameras. If you need to add more cameras later, or add some access control, you can upgrade it by ordering and importing a new license.
Our camera/encoder licensing is on a per-MAC basis, so in a competitive vertical like this, you can minimize the cost of the software by using video encoders. For example, if you went with 4x 16 channel encoders*, you would only be ordering a base license and 4 device licenses despite recording 64 discrete video streams.
Last point I'll make is that if the customer prefers to have a branded appliance instead of putting in a Dell/HP/etc, we have our Milestone Husky NVR series. For up to 32 cameras the Husky M20 would be a good fit.
* Some 16 channel video encoders are effectively 4 video encoders in an enclosure, each with their own IP address and MAC address. These will require 4 licenses as a result. Our supported hardware list has a column showing how many licenses are required for each camera model.