Vehicle Design, R3 vehicle design Options

[GrinderTheTroll]

So I've been researching vehicle design using the Rigger 3 rules and have a few questions and observations maybe you all can help me with.

When designing a vehicle, you use the Design Points (DP) to calc the Design Cost, but use actualy $$ for Customizations, that much is clear. Also, some items cost less CF or $$ when done at design time vs. customization. What I have noticed is adding a "security" item like Vehicle armor, Turrents or fixed weapons would undoubtedly up the Street Index (SI) to 2+ making it more expensive to buy a more or less "stock" security vehicle.

I know my players will want to make some vehicles (since the R3 offers limited designs and lacks some basic types) and I am anticipating them buying "stock" vehicles and then customizing them to avoid high initial SI of security-class vehicles. The trade off is now they must content with not only SI/Availability, Time and Equipment needed. Is there something else I am missing in the DP vs. Custom approach?

What about custom vehicles at chargen? I am considering using the Availability 8 rules for customizations as long as they have the $$ to do what they want (with GM approval of coarse) since chargen time if rather interperative.

Any other thoughts or experiences with R3 vehicle design?

Thanks.