I’m building from scratch. There is a robust open source community and parts are cheap, though they are all from china.
The largest printer at the consumer level is the Crealty 10 S 5 from China for $450. which does a 400 mm cube. Mine will cost $750 but be more industrial quality and do a 500 mm cube.
The low end designs are an inverted T, as is mine, with a moving y axis bed. I will probably have to convert mine to core x y where the bed is stationary if I want to go bigger because the physics of a moving bed get out of control. Live and learn.
Live and learn.
My Robo-3D has a build volume of 254 x 228.6 x 203.2 mm.