I can't offer any comment on re-sale value because it would depend on the price you pay, the rifle, chambering etc. There are however every now and again some very attractively priced second hand doubles available.

Whether it is affordable for you would also depend on how long you would need to hold it for before selling.

However I personally would not have a problem buying a Merkel and using it on a cape buffalo hunt. The main thing I do not like about the Merkel is their stand rear sight, with a notch instead of a shallow V . I would look also at Krieghoff's, Chapuis and Heym's as well. Depending on what your personal preferences are.

The .470 would be a readily re-saleable chambering in a double. As 500grains says the .450/400 would probably also be quite saleable as they are quite fashionable at the moment but probably much harder to find at a good price as a second hand rifle.

Take both the .416 and the double you might get. The .416 with a scope can also be used for longer shots if needed and plains game.

Your danger will be, you might like it so much you can't bear to sell it!