Ok well there are several reasons why to stay away from the £450 models.
First they use Acme lead screw not ballscrews. Acme screw as much more inefficient and high backlash so you accurecy is poor plus they are only 4mm pitch so can't reach the feeds require for cutting plastics correctly. They wear out very quickly and become baggy n sloppy in no time.

Next the Z axis on just about all I've seen won't take the water cooled spindles and are one piece cast units that are very weak. To replace you'll virtually have to rebuild the machine.
The electronics are junk and under powered so won't stand the strain of heavier loads for prolonged periods.

I wouldn't go for any of these chinese machines unless they can take 80mm spindles and have ballscrews with at least 5mm pitch or better still 10mm thou I havent seen one yet that as 10mm.