Oohhh 
Will european HDDs work with the japanese 360?
My european 360 is long dead because of 3RRoD 
like suika said, all HDD's work with each other, any HDD will work on any console, except you might have to change the language to match the console due to annoying text glitches.
(I had a US xbox with 60 GB HDD just swapped the HDD i used to my JPN arcade one, works fine)
if you want DLC you also have to make a JPN xbox live account along with you registered in a valid address in japan. you also need to buy JPN microsoft points, which are very expensive. ($60 for 3500 msp)
you also might need a lot of HDD space, because one stage outfit takes up to around 200-500 mb.
you can basically have a JPN account for im@s DLC and your other account for gaming, as long as the users are all on the same HDD they can all use the addon; except for arcade games.
For price reasons or...?
I bought a Pro system, but my JP Pro 360 is my only 360 so...
if you previously own a 360 console under your country name, just swap the HDD.
but if your getting a 360 for the first time ever, yeah just get the pro.