Since doubles are mostly thought of as being for the BIG DG, then I think a 450/400 is the best choice for a persons first [and last, more on the "last" aspect later].

You can never go wrong with a 450/400.

Even if you later get, or start out bigger, as you get older you will appreciate the 450/400.
Especially if fitted with a scope in claw mounts.

Another great choice is the 9,3x74R. I have one that I have used a great deal. It is a very good killer on all sized game from the small stuff to the BIG stuff.

I have taken giraffe cape buff and elephant with mine.

You cannot go wrong with a 9,3x74R.

Either of these doubles will be a great choice, that will not cause any buyers remorse.

Both have done excellent for me not only in Zimbabwe but in places like Alaska, Canada, Montana, Idaho, and at the deer lease.