The network adapter draws its power from the PS2's power supply, so there SHOULDN'T be any issues, as all PS2s use the same internal voltages.
Hard drives are the same the world over, and the network adapter only takes your standard IDE drives. You can use the official 40GB models, or just slap in pretty much any drive you want up to about 200GB. The PS2 only does 28-bit addressing, so it's a bit limited on size.
would a Japanese ps2 network adapter and hard drive work with an English ps2?