Finding spindle suppliers in UK

First question: If you are designing a cnc machine and you require a spindle, are the air cooled spindles on amazon any good? I am relatively new, about a few months into cnc machining and I haven't dealt with this before.

We are a group of students who have a group and design make project which involves the design and make of a double spindle 6 axis CNC machine which is required to be multi functional, meaning that it should perform more than one process. There are two processes which include double sided friction stir welding of a thin sheet of lead and double sided incremental sheet forming in the shape of a truncated cone. We are looking for a milling spindle DC motor which can be tailored into our design. We would prefer a diameter of roughly 50mm but this can be adjusted. An ideal length of between 50 and 150mm but again it can be adjusted based upon what is available. Due to the two processes, it is required that the spindle is carefully controlled in terms of its torque and speed. Therefore, the spindle will ideally have some form of speed controller to vary the range of RPM from about 50 rpm up to at least 1000 rpm. Voltage and power isn't too much of an issue so long as an appropriate safe power supply is readily available from major suppliers and the spindle can meet torque and speed requirements.

Thanks for reading and useful informative replies are welcome

I think you'll struggle to find something running as low as 50rpm to 1000rpm, that's more like DC Servo/Controller and 3-Phase/VFD territory.
You might also struggle with bearing damage from side loads from metal forming, many spindles aren't designed for that.
You might be able to do this using a 3 phase motor with a custom shaft, tool holder and front housing?