I have one of the 6040 machine you mentioned. Still in the process of getting it set up
Mechanically it's OK. A few rough edges to construction but nothing major, The controller is very basic and in my case, had a bad driver for the X axis. Spare boards are available but it's a pain to have to source one and install it, especially in a brand new machine. Forget about warranty as it will require that you send any faulty item back to the seller and to pay shipping on the replacement - not cheap!
The only thing I have concerns with the machine is the limited Z travel, only 70mm or so. You may want to consider this if larger (thicker) items are to be machines. This also limits the size of items that can be used with the 4th axis
I'm considering a 4th axis for mine as it will help offset the freight cost of buying a new driver board.

Due to the dead driver, I haven't been able to fully test it. As I have a Gecko G540 controller I'm going to use it instead of the controller supplied.
It's not a good time of year though to be tracking down the necessary bits and pieces I need, everything is shutting down for Christmas.