Hey atm guy, I also ATM and found that I hate the storage of the stock Nissan nv200.
Lately I've been using milk crates and rubbermade boxes for storage in the back. I also use have a cage in which I use binder clips, bungee straps and caribiner clips to attach or hang stuff off it.
As for the speakers, this and other Nissan's sometimes
Use 2 ohm speaker(don't ask me why) but with the two speaker set up I've managed to get away with 4 ohm speakers.
The speakers are mounted some weird fashion, with only 3 screws, not your usual 4 screws so an adapter might be necessary. Any 6.5 should be fine.
Also the inline connector plug is the one that the Sentra/frontier uses IIRC.
I've been wanting to wire a second set of speakers in but as mine is a work van also, and I'm not sure what size the pin is on the harness I'm holding off on it .
If I can manage to get some time to do a write up of the speaker installation I'll post one up, it's simple and straightforward with the right tools.