i don't know about poland but over here (netherlands), 650s are somewhat under-appreciated compared to the 750 and 500/550. that makes them and parts for them a bit cheaper.
all those bikes you find... try them all! that will give you a direct impression of the differences between models and individual bikes, will make you better at deciding which is the right one.
generally i agree with the previous advice to look for one that's in as original a atate as possible, and as un-molested as possible.