Need help with deciding on an SBR...

We are getting a firearm for a team member. In order to further train him in the sorcery that is the ATF/NFA process, we are going to invest in a firearm that he can take from a firearm and make into an SBR.

We are hoping we can switch the roles and get some help from r/NFA. What was your first SBR? Do you have an SBR build that you had fun making? Any SBR suggestions?