Apparently the JG MP5s are all plastic-bodied, but still not too bad despite that. Not particularly study but also not particularly weak.
The Cyma MP5s seem to get good reviews, but they're about £230 for an SD.
Another thing to bear in mind for CQB, the SDs are about 4" longer than a regular MP5, if that matters to you.
They still look cool as hell and you can hide a tracer inside the silencer. ??