It is very unlikely that any harmonica will overblow out of the box. You will have to adjust the reeds. If I had to choose one harmonica to learn overblowing, I would probably go for the Golden Melody. Why? Well, I've noticed that the Golden Melody is very responsive to overblows. As far as I know, the great Howard Levy learned to overblow on Golden Melody harmonicas. In any case, you may have to adjust the reeds a bit to make any harmonica overblow.
Although a C harp is good for overblows, you can also get a Bb or A harp. I have a special 20 in A which overblows very easily.
The 4 hole overblow is a little difficult to play on low key harps (such as the key of A or G) though. However, it's easy to get some of the overdraws on low key harps.