If you really want a GBBR that work with not too much fiddling out the box the KJW M4 would be a great choice.
Got mine for more than a year now, gamed it last summer until october and gamed it last week in cqb in a mall that was not heated and worked real well.
There are 2 upgrades you should get for this gun, the cradle velocity reducer and the cradle piston. These 2 upgrades together cost about 60$.
There is a new version bolt for this gun but it seems to not be as reliable as the version 1 bolt. If you have any question about the KJW working or anything there is a big thread on this forum about it.