Gear rental in Japan is no different than anywhere else- dive computers here are the same as anywhere else in the world- except for few models that are only released in Japan.

Wetsuit and BC sizes at most places will be fine for most foreigners unless you are really tall or quite fat. You will probably need a size larger than your regular size those for med use a Japanese large etc...

The only other problem I have encountered is with booties for people with large feet US12 or greater as most places do not have rental booties that size. The best thing to do is contact the shop you are planning on diving with and ensuring they have your size.