The original plan was to upgrade the existing 430 and get the STC as mentioned but when you add another £3k plus to replace the second radio you are almost in the same price bracket as simply putting in a 430W and buying the STC that way.
Do both 430's have to cross fill data?
As mentioned above I have already come across 'offers' of well priced units asking for money to be sent to people with nothing more than a gmail account. There was one on last weeks UKGA add list. Beware!!
BB