2. Your "budget" of £120 is not going to work. You can barely buy a single part, much less a whole PC for that.
For instance, 16GB DDR4 is ~£150.
3. Your chosen "SSD"? 8GB? Can we see a make/model of this thing?
You might be able to find a 3-4 generation old Intel based workstation system for not much more than your price point.
But it will not be "incredibly fast".