Welcome!
Tricky, I’ll narrow it down to top 3
Apex Flex
FlexSecure
FlexM1
NExT and xMagic
2in1 between them they cover MOST possibilities
These will likely be your most popular
xG3 for people who want Magnets
xSIID for people who like blinkies 
FlexNT for Business Card sharing
For those that like online security:
Apex Flex - Off the shelf - app supported digital identity
FlexSecure - “Hacker” version of the Apex Flex - digital identity
Equally as good as each other, but depends on the user preference ( I have both )
My first suggestion for you, to have as a useable tool, shows how the technology can work, great performer, and effectively an implanted business card ( Maybe tax deductable 
)
I would also suggest you get either a Proxmark3 or a Flipper Zero, depending on your knowledge in the area.
This would allow you to test implants immediately after install, and you could offer a full service of install and reading / writing most credentials for customers ( there are limitations )
- The Flipper Zero is a user friendly, quite capable device
- ProxMark3 - more capable than the Flipper, but not as easy to use.
Finally, I would suggest you stock some RSP’s, these will make phones with shitty NFC antennas, far more capable of reading NFC implants, and phones that work well, even better.
You can get those through DT store by following the RSP link or from Hammys store
Also, if you have a customer come in, and they have questions, just reach out and somebody SHOULD be able to answer their question if you cant.