Hope you do not mind me jumping in. I am in the same situation and have been searching for something with basically the same criteria. Your information is great as I have not read about the advantage of having a mid length gas system. I have been looking at the Rock River Elite Operator (mid length) they have on sale.
http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=397
Then I find the Del-Ton DTI-4 A3 for only $670.
http://www.aimsurplus.com/product.aspx?item=F1ATDELTONA3&groupid=11
Three gun is something I want to do, but it will not be a major concern. It seems the local club has a habit of canceling many matches because it is either too hot or too cold.
Thanks.