Here is a description of Poirot that should give you an idea of what his taste in interiors might have been!
The famous sleuth loves the finer things of life and luxury, including exotic drinks (he loves sirop de cassis), exotic locales, excellent served food, the theater and other arts, and very comfortable hotel rooms. He loves the indoors and avoids extreme weather whenever possible. Poirot loathes dirt and disarray, and favors order, method, and symmetry (Whitehaven Mansions is symmetrical, his eggs for breakfast always need to be the same size, and even the books on his bookshelf are arranged by height!).
This is according to- http://www.poirot.us!
We can thank Jeff Tessler for the fabulous set design on the show, he gave the apartments true Poirot Personality!
Lets take a look at some of the pieces I found that I think good old Poirot himself would have considered buying for his own apartment!
If you really want to learn and see everything about Poirot then visit Poirot Chronology,
the interior shots are credit to that blog and he also has quite great opinions on the style during the time of Poirot!
All these fabulous finds are courtesy of 1st Dibs! Search 1920's or Artdeco and you will find a trove of gorgeous pieces. You can also search Ebay and Etsy as well they always have fun finds too!