Everyone has good feedback from smartgunner, though I have not personally used them. I would assume based on what I have read that you will be getting a good, well put together unit though. Haven't heard anyone reporting a real fisting from those folks, so I would imagine they will do you right.
Mid length gas system?
No real opinion to be honest. It will work just fine, though it will be VERY SLIGHTLY
less accurate than a full length system. Your BCG/Upper Receiver Action is going to get a bit dirtier when shooting because the gas that is being collected is closer to the chamber where the round goes off. More powder has yet to completely vaporize and so you will get a bit more crud in your upper action.
Is this a real concern?
If you were shooting 10,000 rounds a week in a combat situation, yes. It would be a concern. If you are going to be shooting 200 rounds every other weekend and plan on doing regular maint on the weapon, no. It will never effect you.
One recommendation I would add to your cleaning procedure would be to use the pipe cleaner in your gas tube more often than what folks would consider "normal" just to keep the amount of build up down.
Once again, it's not a real concern for the "average" type shooter, but it is a good maintenance policy.
All in all I think you have assembled a great stick and it should perform well as long as Mr. Murphy doesn't sneak by.
Looking forward to seeing some finished pictures and a range report telling us how she worked for you.