I have the 124G and a Mamiya M645 (not the J, not the 1000S, but the plain M645).
I think 200 euros is too much for a J series. It was the entry level 645, and lacks a few features, like the MLU, and the dual shutter releases. There are more things, but off the top of my head, those are what I remember.
The 124G is a great camera, light, easy to use, built in meter, great lens. But it's (IMO) limiting in that you have one option for a lens. There is also the risk of jamming the shutter using the self timer with the sync set to M. And you have to watch your parallax in closer images.
Personally, I prefer the Mamiya. I like the SLR, I love the interchangeable lenses, and it's super close to being a system camera. Different finders, available TTL metering, power winder, and pre-loadable film inserts. Just some niceties that the Y-124G doesn't have.
The other thing is; which format do you prefer? 6x6 or 6x4.5?
With that said, for $200 US (about 200 Euros I think), you can get yourself a nice M645 or 645 100s with the 80mm, and probably have some left over. The lenses for the Mamiya are great, and lucky for us, are really affordable. I think the most expensive accessory is the waist level finder with the sports viewfinder.
My recommendation is a Mamiya M645 or 1000S, fwiw.