Sam,

When the manual was written, the 3000mAh NiMH was likely the hot cell. Now, it is up to 3700mAh NiMH. Any one will work fine but you'll likely get a better price on the older cells from 3000-3300mAh. The big difference is in the battery technology jump from NiCd to NiMH. In the future, you can jump again from NiMH to Lithium.

The 8.4v rating of the motor is only nominal operating voltage, not maximum.