If he bases his plans on 9 mm, may I suggest eventually adding a Kel-Tek Sub 2000. It is available to match the Glock magazine if that is in the inventory.
Not a high-accuracy weapon, but has a unique way to store small and quickly put in use.
It is on my list, but not yet in my inventory. They are very hard to find these days without overpaying.
I had the SUB2000 in .40SW/Glock22 mags.
I found it completely un-interesting after a day at the range.
Shitty trigger, lousy grip ergonomics and crappy sights.
Not particularly accurate.
I got bored and sold it about two months later.
I’m still waiting for a REAL Glock carbine, similar to the fantastic Beretta CX4 Storm carbine. The folding nature of the KelTec just makes you give up too much (decent optics, for one).