Probably stating the painfully obvious, although the centre heights are nominally the same, any tooling that clamps to the bed of the ML4 will not clamp to the ML7 because the beds are completely different.
What the ML4 and the accessories would fetch would probably go a long way towards a Chinese lathe.
A mini lathe is about the same size as a ML7, and if new should not have the wear problems intrinsic with any used machine. But since your horizons are already expanding., the SC4 comes to mind. (The changewheeel set includes a 127T gear, so changing from Metric to Imperial threads should cause no problems. )
You will be changing from an Imperial machine to a Metric one., and although the Myford Vertical Slide allows milling, it is not, in my experience, particularly rigid.
So obviously, I am going to advise a dedicated machine for milling.
It would be good to have machines by the same manufacturer, again my view. And Sieg is the name that first comes to mind.
The over riding factor in this apart from space, has to be budget, of course.
If it can be stretched, good choices now will avoid changes, and expenditure, at a later date.
The choice has to be yours, despite all the preferences expressed by us.
Howard