Low carb Metabolic Drive should be the protein of choice by far. The flexibility is there to couple it with fruit/veggies for a P+C or nuts/seeds/oils for a P+F.
If you want slow digesting use a casein blend protein, like Metabolic Drive. Whey itself is rapidly digested and absorbed, though the nat PB will slow this some.
I would only add the fruit it the meal replacement is early in the day.