Most realistic GBBR option you could go for is the Inokatsu M4, however it is very expensive as it is one of the most well made GBBRs out there.
There is also the G&P WOC system which is based off the WA system. Most of the GBBR systems of there right now are based off the WA system except for WE and KJW.
Companies like G&P WOC, VFC and King Arms all base their systems off the WA system.
you could try those.
VFC HK416 Custom
Custom Bowie Tactical x SAI Glock 17 (TM Base),
Custom SAI BLU Glock Build (Timberwolf Base)
Custom Zev Noveske Shooting Team Glock 34
Custom Zev Dragonfly G17 (Project) (In progress)