Tornado mer is not a good design, don't bother... it will take you some tricky engineering to get the blade lower than quite a few of the wedge bots out there, and that isn't done then your robot just won't be able to do anything. The best bet is to build something like Hazard, basically a wedge robot with an overhead bar... the wedge helps you get anything lower than the bar up into it.

If you still want to build it, heat bend some UHMW into a ring around a former and weld the ends together... it welds pretty nicely with an appropriate plastic welding setup and will be a lot cheaper than milling the ring out of a plate.

If you're doing something like the original design you wanted, the best design would be a fbs/shell spinner... to build the shell there are a few approaches: you can get it rolled out of sheet and welded before balancing, or just mill the whole thing. The sheet approach is most economic but both will be expensive, milling very so unless you have a mate with a bridgeport or something!

Also driving an overhung blade is tricky, big bearings and a hefty mount will be required which will take more machine work...the only off the shelf solution is to buy a right angle gearbox for a magmotor from team whyachi robotics (google them) and use a magmotor to spin it - the two combined will probably set you back 700 quid and that's without a blade. Failing that you can machine your own bevel gearbox (use hardened spiral bevel gears and hopefully some kind of clutch, either where the blade mounts or the input or both) or drive it with another approach - perhaps push fitting a pulley over the can of a brushless and belt driving it from that?

Either way... a spinner of that kind is quite an undertaking to build in any way that will give you something effective.