Will we see MS release the 650 & 650XL to co-incide with the 'coming-soon' Windows 10 mobile? If not when will we see a lower end pure Windows 10 phone? (Rather than a software upgrade for the 640)
No, Windows 10 Mobile launch won't coincide with launch of Lumia 650/650XL.
As per rumours, 650 and 650XL would come in MWC in March, 2015. Lumia 640 would get Windows 10 upgrade somewhere around November/December 2015.
I don't know for sure if 650XL will be announced, but I guess if it will come, it probably would have following inmprovements: Snapdragon 410 or 412 or even 415 processor, so new Graphics processing unit, new IZAT C GPS, on-die LTE(4G no 3g only models), improved performance due to new processor, Windows 10 Mobile OS on launch, Gorilla Glass 4, 16 gb internal memory, maybe 1.5 gb RAM.