So I want to challenge myself more and I was thinking about getting a GBBR. Looking at: WE xm177 (son tay/macv-sog) set up. A Tokyo marui mk18 or  URG-I.  KWA the LM4. Any opinion or experience would be appreciated.

Not the same guns, but I own all of those platforms. The Tokyo Marui ZET system GBBR is the probably the best performing and has the most aftermarket support. I wouldn't get the MK18 Mod 1 because the handguard and barrel nut is designed differently and requires different tools and difficult to remove if needing to replace hop-up parts. If you wanted the shorter outer barrel, get the CQBR.

Edit: Re-read your post and looks like you're also interested in the Tokyo Marui URGI GBBR. A friend of mine has one and has no complaints.

Thanks for the insight, the TM URGI is gtg thanks for the insight. On a side note I've had HPA (both wolverine and p*), AEGs from probably every major manufacturer in airsoft but I've never done the GBBR or had any AEGs with blowback (like the NGRS, KWA or Bolt EBBs) and I've been in the hobby since 2001 so I figured why not change things up.