Open bolt would be a better choice. New or used off of the classifieds. Used ones tend to have upgrades already done to any problematic parts.
I know you're scratched AEGs off your list but a good AEG will be far more reliable than even the best GBBR, in case reliability is a big priority.
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)