i used to have a pa50. good arranger, very good sounds and styles, but indeed the build quality was not the best. the reason why i sold it (one of the reasons) was the very anoying background noise coming out from line out conection. if i was going to record more than three audio tracks, the noise so loud that i ruin all my work.
anyway, that might've been a singular problem.

on the other hand, exr7 gives you good keys, 76 of them and pretty good sounds too, but not comparable to pa50. u could use it as a midi controler, it has good midi impl. (not the best anyway) and take advantage of that.

if i were you, i'd wait until e50/60 will be available.