I have a long table sieg sx1 under diy CNC control, it makes for a fun project and it is a game little machine. A bit bendy for steel work perhaps but certainly capable of munching aluminium. It is also a pain in the arse.

With respect to your post:
The max spindle speed, 2000rpm, is almost okay for aluminium but I think you need something quite a bit faster for plastic.
Y travel is only 140mm so for a 250mm piece you would need to rotate the work half way through the process.

With respect to CNC:
Z control is random, sometimes the head will dip down a fraction when the screw is driving it up and sometimes the head will stick for up to 0.5mm when commanded down. The ways, guide rails for want of a better description, on the back of the head are just too short, the adjustment way to twitchy, there is no real, long term fix.

The X and Y backlash adjustments do work but are not designed for the sort of continuous back and forth movements you end up asking during CNC operations. Over an 8hour job you'll be able to see the errors starting to creep in.

The ways all around, metal to metal, dove tails with gib strips are likewise not designed for 8hours of continuous motion. The X especially starts to show slop at the end of a hard day.

So, £700 - £800 for the X1 + motors + drivers + home made hardware, quite a bit more if you buy one of the commercially available kits. I'd seriously consider some other options.