IFR or VFR? If the latter, from what I've read here in the past, you need some sort of prior authorisation. I don't know how easy it is to get, but I believe posters Van Horck (and possibly IO540?) had some details.
Are you planning on doing it all in one day? A quick calculation shows that it
might be feasible if you depart very early. Assuming EGSR opens at say, 05:00 local and you can be in the air half an hour latter, you could with luck make LRBS by 19:00-ish local.
Sensibly you would require two fuel stops along the way, e.g., one in Germany & one in Hungary, although with careful planning on a good day you might be able to get away with just one stop in Southern Germany, but that appears to me like pushing it a bit.
This is all assuming you are VFR. Needless to say, flying IFR would be recommended for this trip--for a start, you do not have to worry about sunset (I very much doubt night VFR is permitted at all in Romania), on top of that, your routing would be fairly direct (FPP shows only a 7% excess over the GC route if flying airways). Lastly, life's just so much easier flying IFR
I do not have any VFR charts for Northern Germany with me at the moment, but from what I can see on the charts I do have (ED, LK, LO, LH) and from previous experience in the South, you should not be hitting anything too complicated in terms of airspace at least until you enter Romania (I do not think there are any VFR charts at all for it). Even so, if going VFR I would plan on a) making sure you can fly in Romania at all, and b) making my way there over two days.
As for the transponder bit (which is what your question was about after all
), you would have to check the relevant AIP, or perhaps easier, call a couple of ATS offices in the countries concerned. I had mode S already last time I was in the area about six months ago so not familiar with specifics.
Hope this helps