What Does Buying an Insurgent from Warstock Require?

You can buy the Insurgent without owning any property, it's a Pegasus vehicle you can ask for, by calling Pegasus on the phone.

Source

There's 3 Insurgent versions, each of them will require a different place to save it:

Insurgent Covered

This will be unlocked after completing the Humane Labs Raid on PS3 and Xbox 360, you can buy it anytime on the PS4, Xbox One and PC versions.

It can be saved on your usual Garage.

Insurgent Pick-up/Armored

The unlock requisites are the same as the normal insurgent.

This is a Pegasus vehicle, so you will have to pay GTA$100 every time that you want to request and use it.

Insurgent Pick-up Custom

You will need a Mobile Operations Center to get a Pick-up Custom.

On the vehicle workshop you can convert an Pick-up into an Pick-up Custom for GTA$202.500, and after that you should be able to save it in your usual Garage.