A better question might be related to your intended use and size requirements. An 8x12, although not substantially bigger in rated capacity to the 7x12, is a substantially better machine, if for no reason than it outweighs the 7x10 by a factor of 3:1. That in and of itself makes it a more stout machine. Also to consider is if you want or need to be able to move the lathe by yourself. The 7x10 is no problem, but the 8x12 requires equipment to move safely.
Regarding conversion, member roncat (I think?) did an amazing conversion on an 8x12. However, I don't recall if it was capable of running manual or not.
I am working on a 7x10 budget conversion that retains full manual control. My conversion is one of the only I've seen that doesn't utilize ball screws. MY BUILD
Threading is doable on a cnc converted lathe, but you'll need a spindle encoder and the appropriate software to run the lathe. Apparently the free version of Mach does not support threading.