I only have experience of Garmin eTrex models....
The screen is too small to use it as a replacement for a map, but it is a great safety tool. I find myself registering locations I visit frequently and then jusr using the "Go To " function which just tells me the direction and distance, I pick the roads myself.
You can plan routes by using the PC software that is included.
If you buy an english one, you will need to purchase additional Third Party Japan map. All locations will be in english. Alternatively purchase the Japanese one which includes the map but you had better learn Kanji.
Lastly there is the mounting issue. I use the Garmin Bicycle mount, but it is not too robust. Alternatively just leave it in map compartment of your tank bag.
Actually I have a couple of units that I don't use. A yellow basic etrex with no map display functionality or an eTrex Legend C with mapping , both english. Let me know if you are interested.
I might be interested in one of those units. I will see if we can meet when I get back from Hiroshima.
Phil