Dear Mike
I agree WHOLEHEARTEDLY about the Gounod. Singing that sanctus the priest could say another Low Mass in between. Mr Carey is absolutely right that the music MUST NOT hold up the 'action of the Mass'. I'd be in favour of a Palestrina Mass which is always "churchy" without being maudlin and the 4 voice should fit the bill. I'm not overly keen on the 'mix-&-match' idea of bits from several masses although your Dad will remember that being regularly done in the glory days of St Michael's choir.
David