PDW's are generally less than 10". Ten inches & up are generally classed as CQB.. now that the semantics are over with.....
If you are going with this short a barrel I would not recommend building from parts your first time out unless you have done your homework and have some trigger time / understanding with the AR platform. They can be finicky and problematic if the parts are not truly compatible / in tune. "Factory" built uppers of this size are generally a lot less trouble.
DI systems are much more common, cheaper to buy but a lot dirtier than piston driven systems. Piston systems are generally more expensive but a WHOLE LOT cleaner and the gun will run a lot cooler as a result of the design.
Barrel attachments.. I run a YHM flash suppressor on my 7.5"... yeah I love the boom! You can get various flash suppressors to cut down on muzzle flash (which is significant in these SBR's / AR pistols). You can also, if you want to be more considerate of other shooters,get devices like the Flaming Piggy or Dlask to direct noise more down range & directly in front of the shooter to cut down on the noise. It's somewhat a matter of personal preference really....
Buying a complete upper.. for range use (which is what 99.9% of these get used for) any of the brand names will be a-ok in most circumstances. The "top tier" uppers may have some bells and whistles which may or may not be what you want. I'm not going to recommend any specific brand because that will undoubtedly start a flame war so..... Buy the best you can afford; worst case you can always upgrade after the fact.
