I need to customise Zen to allow tax rate to be selected by looking at product and customer information to allow use of composite vat rate.

eg

Product A purchased alone = 17.5% tax
Product A purchased alone by Registered disability customer = 17.5% tax

Product B purchased alone = 17.5% tax
Product B purchased alone by registered disability customer = 0% tax

Product C Purchased alone = 17.5% tax
Product C purchased alone by registered disability customer = 17.5% tax

Product D Purchased alone = 17.5% tax
Product D purchased alone by registered disability customer = 17.5% tax

Product A and Product B purchased = 17.5% tax
Product A and Product B purchased by registered disability customer = 6% tax

Product A and Product C purchased = 17.5% tax
Product A and Product C purchased by registered disability customer = 6% tax

Product A and Product D purchased = 17.5% tax
Product A and Product D purchased by registered disability customer = 17.5% tax

17.5% is standard tax
0% is available on certain products for certain customers otherwise it is 17.5%
6% is company composite rate when combining certain product types with qualifying additional products.

So i need to add info to customer to store if they qualify for reduced tax rate.
Info to products to say IsType A or IsTypeB or ...
or could flag as IsTypeA, Qualify0, TriggerOnly.

Two questions:
1. Is this possible without completely hacking Zen about and having to rewrite it every time there is an update

2. Where do i start!?