If Red Bull build their own engine, they will not build an engine. I will be a badge job on an engine build by a manufacturer, either one already in F1, one entering F1 soon, one who have previously been in F1, or one who have wanted to stick their toes in the water, but will try do so in the relative anonymity of a badged job (VW?).
I would not be surprised if it was a re-badged Renault becoming an Infiniti, I have a feeling Red Bull think they can do a better job with some of the issues than what Renault have done, and we have already seen how they chose to run configurations and settings of their own this season.
Most likely not something which will happen until at least 2016.