Well, Belch's power is always 120, so if you're worried about that, then don't be.
Sistrus Berry is simple and effective. Not only does it increase the Pokemon's longevity, it gets activated fairly quickly, meaning you can use Belch early on and for the rest of the match. Oran works too, but 10 HP is a little pathetic.
Lum Berry is also simple and effective. Lum is used widely anyway, so why not pair it with a powerful move? The berry itself is extremely useful, and can save you in many battles. However, status moves aren't always used, or if they are, not necessarily early on. This means you could be sitting with a redundant move for some time.
Chesto Berry can be used in junction with Rest and Belch. You heal yourself completely, wake up straight away, they unleash a strong attack!
Shuca / Passio / Charti Berry's reduce Magmortar's Ground, Water and Rock type move Respectively. Increases his survivability before retaliating.