I have the same reservations about David Walliams as you do. A lot depends as to whether or not he can restrain his personality from taking over the role. Jessica Raine should be alright though.
Just found out that the 6 episodes are 2 3-part stories, not 6 stories. First is N or M and the second is The Secret Adversary, both of which are book adaptations. There were two other books and a collection of short stories so a lot less than Poirot and Miss Marple.