In my opinion, the NV200 has zero potential so why modify it?
I've been hot rodding since I was 12 years old. Our 5.0L Mustang, our Dodge Ram truck and my wifes Acura TSX are all modified and lowered.
The NV200 has a really cheap and basic suspension. Thats why it handles so poorly.
The front suspension is a very basic macpherson strut with soft springs, a small front sway bar and small brakes.
If you lower the front you will likely not be able to align it correctly since there is no provision for adjusting camber.
The rear suspension is a solid axle with leaf springs with no rear sway bar and tiny drum brakes.
There is very little suspension travel and the shock mounting is poorly designed.
You really can't lower the rear with spring changes since then the axle would be riding on the bumpstops constantly.
But.....
I have spent time and lots of money on the website.
www.detroitspeed.com
Just by chance, I noticed that they slightly modded an NV200 van.
https://www.flickr.com/photos/detroitspeedinc/sets/72157688826731450
These guys are serious. They don't fool around..
You can see that they added a rear sway bar.
It doesn't look like they modified the front suspension at all.
They lowered the rear suspension by modifying the rear axle spindle location points.
This was not easy but this maintained the rear suspension travel and load capacity.
But then you still have a slow underpowered van that handles poorly! Good luck.