There aren't a whole lot of options out there.
Special Weapons/Coharie Arms/Whatever they call themselves now if they are still in business used to be the most widely available, and most universally hated, of the clone manufacturers. Some of their guns came out good, others came out with problems.
There was a company in Ohio that also made them - can't remember their name to save my life anymore though. I think a lot of their products were SW flats that they folded in house and built with better QC.
You can still find flats out there that you can roll yourself and do a build, but building an MP5 isn't in the same ballpark as building an AR - you need to have actual tools to build an MP5 (press, feeler gauges, welding machine, etc.).
I have a Special Weapons - it''s an older model and I haven't had any issues with it, but it's definitely not HK either. Fun as hell to shoot, probably going to SD it as one of my next gun related projects.