Since you are adding fruit juice, brandy, sugar and maybe sparkling water, you definitely need not buy anything expensive.
I'd stay away from cabs and (sorry vera) zinfandels because they generally are pretty robust and are not really "light" reds.
Because you are going to adulterate the wine so much, I really don't think it matters that much.
I made sangria almost every weekend using a couple cases of $2 wine I bought at the local liquor store. It was delicious!
Less is not more. More is more and more is fabulous.